How do I link librt and libpthread to a new layer in Yocto?

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Where can I find documentation on aarch64-poky-linux-ld?

Please ignore sets of outer double-quotes. They seem necessary to get "StackOverflow" to accept my input.
My layer is "meta-oca-so", its package is "oca" and its recipe is oca_1.2.7.bb.
I revised my "oca makefile" to fix the previous and some linker errors to do with invalid link flags, and I'm now getting the full 29 "oca" .a libs to link together when I do "bitbake oca". I didn't have to do any clean while changing the link flags and redoing "bitbake oca".
Now when I do "bitbake oca", the error messages are:

~/Yocto/imx-yocto-bsp/build-wayland/tmp/work/aarch64-poky-linux/oca/1.2.7-r0/oca-1.2.7/Obj/linuxApp/Release/OcaProto
aarch64-poky-linux-ld: cannot find -lstdc++
aarch64-poky-linux-ld: cannot find -lrt
aarch64-poky-linux-ld: cannot find -lpthread
aarch64-poky-linux-ld: cannot find -ldns_sd

Focusing on "librt" and "libpthread" in this question, I added these lines to my "local.conf":

IMAGE_INSTALL_append=" librt"
TOOLCHAIN_TARGET_TASK_append=" librt"
IMAGE_INSTALL_append=" libpthread"
TOOLCHAIN_TARGET_TASK_append=" libpthread"

But it didn't clear its two link errors when I redid "bitbake oca".

When I tried the overall "bitbake imx-image-multimedia" I got these messages about "librt":

NOTE: Resolving any missing task queue dependencies
ERROR: Nothing RPROVIDES 'librt' (but /home/james/Yocto/imx-yocto-bsp/sources/meta-imx/meta-sdk/recipes-fsl/images/imx-image-multimedia.bb RDEPENDS on or otherwise requires it)
NOTE: Runtime target 'librt' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['librt']
ERROR: Required build target 'imx-image-multimedia' has no buildable providers.
Missing or unbuildable dependency chain was: ['imx-image-multimedia', 'librt']

It's hard to find much info about "rt" because this two-letter combination is not statistically improbable.

Similarly, when I commented-out the two "librt" lines from "local.conf" and retried "bitbake imx-image-multimedia", I got these messages about "libpthread":

ERROR: Nothing RPROVIDES 'libpthread' (but /home/james/Yocto/imx-yocto-bsp/sources/meta-imx/meta-sdk/recipes-fsl/images/imx-image-multimedia.bb RDEPENDS on or otherwise requires it)
NOTE: Runtime target 'libpthread' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['libpthread']
ERROR: Required build target 'imx-image-multimedia' has no buildable providers.
Missing or unbuildable dependency chain was: ['imx-image-multimedia', 'libpthread']

How do I obtain "librt" and "libpthread"?
Do I need to add a layer each to build them? Will there be a "dependency hell" tree of supporting packages needed for each of them?

Here's the relevant makefile component, makeOCA.inc. There are 63 "make" files of different descriptions and at different levels in the OCA project. It's pretty long, at ~1070 lines. I snipped the sections for non-Linux systems; I'm only concerned with linuxRelease here.

#  Project             : OCA
#  Module              : Multiple components
#  Description         : Include file for C / CPP makefiles.
#

################################################################################
# Environment variable checking
################################################################################
#
# If one of the variables below is not set,
# calling make will result in a "missing separator" error.
#
ifeq ($(CAP_HOME),)
  variable CAP_HOME is not set
else
  CAP_HOME := $(patsubst %/,%,$(subst \,/,$(CAP_HOME)))
endif

################################################################################
# Variables
################################################################################
ifeq ($(NAME_MKE),)
  NAME_MKE := makefileOCA
endif
ifeq ($(IGNORE_MAKE_ERRORS),Y)
  PREFIX := -@
else
  ifneq ($(DISPLAY_MAKE_CMDS),Y)
    PREFIX := @
  endif
endif
EMPTY :=
SPACE := $(EMPTY) $(EMPTY)
#
# Only set variables when the makefile is called with an actual target
# (not a phony one).
#
ifneq ($(PLATFORM),)
  ifneq ($(RELEASE),)

    # Project home
    PRJ_HOME := $(CAP_HOME)

    # Src home
    SRC_HOME := $(PRJ_HOME)/Src

    # Include directories
    ifneq ($(USRINC),)
    INCDIRS := $(subst ; ,;,$(subst \,/,.;\
        $(SRC_HOME)/inc;\
        $(SRC_HOME)/lib/inc;\
        $(SRC_HOME)/common;\
        $(SRC_HOME)/common/OCA;\
        $(SRC_HOME)/common/HostInterface;\
        $(SRC_HOME)/platform;\
        $(SRC_HOME)/platform/common;\
        $(SRC_HOME)/adamson;\
        $(USRINC)))
    else
    INCDIRS := $(subst ; ,;,$(subst \,/,.;\
        $(SRC_HOME)/inc;\
        $(SRC_HOME)/lib/inc;\
        $(SRC_HOME)/common;\
        $(SRC_HOME)/common/OCA;\
        $(SRC_HOME)/common/HostInterface;\
        $(SRC_HOME)/platform;\
        $(SRC_HOME)/platform/common;\
        $(SRC_HOME)/adamson))
    endif

    # Output directories
    OBJDIR := Obj
    OBJINT := intermediate
    OUTDIR := $(subst \,/,$(PRJ_HOME))/$(OBJDIR)/$(PLATFORM)/$(RELEASE)
    ifeq ($(GINTDIR),)
        GINTDIR := $(OUTDIR)/$(OBJINT)
    endif
    INTDIR := $(GINTDIR)/$(NAME)

    # Tools
    SD := CC -M -MP -march=native
    SE := sed
    RC :=
    ifeq ($(OS),Windows_NT)
        RM := erase /F
        MD := mkdir
        RN := ren _PATH__OLDNAME_ _NEWNAME_
        RD := rd /s/q
    else
        RM := rm -f
        MD := mkdir -p
        RN := mv _PATH__OLDNAME_ _PATH__NEWNAME_
        RD := rm -rf
    endif

    # Commands
    ifeq ($(OS),Windows_NT)
        IFEXIST    := if exist _FILE_ _COMMAND_
        IFNOTEXIST := if not exist _FILE_ _COMMAND_
    else
        IFEXIST    := if [ -e "_FILE_" ]; then _COMMAND_; fi
        IFNOTEXIST := if [ ! -e "_FILE_" ]; then _COMMAND_; fi
    endif

    # Other OS dependent variables
    # Work around to make sure only a single backslash is set
    ifeq ($(OS),Windows_NT)
        PATH_SEP := $(subst \\,\,\\)
    else
        PATH_SEP := $(subst \\,/,/)
    endif

    ifeq ($(PLATFORM),winntApp)

      CXXFLAGS :=
      RCFLAGS := -I "$(subst ;," -I ",$(INCDIRS))"


### linuxRelease ###############################################################

      ifeq ($(RELEASE),Release)

        # Tools
        # CC := gcc

        # Determine the extensions for the target platform
        OBJ_EXT := o
        DEP_EXT := dep
        EXE_EXT :=
        LIB_EXT := a
        DLL_EXT := so

        # Lists
        OBJS := $(patsubst %.cpp,%.$(OBJ_EXT),$(SRCS) $(LINUXSRCS))
        OBJS := $(patsubst %.c,%.$(OBJ_EXT),$(OBJS))
        DEPS := $(patsubst %.$(OBJ_EXT),%.$(DEP_EXT),$(OBJS))
        LIBS := $(LIBS) $(LINUXLIBS) $(EXTERNAL_LIBS)
        LIBS := $(addsuffix .$(LIB_EXT), $(LIBS))
        LIBS := $(addprefix $(OUTDIR)/, $(LIBS))

        SDFLAGS := -O2 -nostdlib -fno-strict-aliasing -ffunction-sections -fdata-sections -c -Wall -Wno-unknown-pragmas -x none \
                   -I"$(subst ;," -I",$(INCDIRS))" \
                   -D"NDEBUG" -DTARGET_LINUX -D"CPPUNIT_TUI_VERSION" -D"__STDC_FORMAT_MACROS" \
                   $(foreach USRDEF,$(USRDEFS),-D"$(USRDEF)") \
                   $(foreach USRUNDEF,$(USRUNDEFS),-U"$(USRUNDEF)")
        SDCFLAGS :=
        SDCPPFLAGS := -std=c++0x
        CCFLAGS = $(SDFLAGS) -fexceptions -fvisibility=hidden -march=armv8-a -mtune=cortex-a53 -o $(INTDIR)/$@
# That's for Linaro and the Snapdragon410 (Qualcomm APQ8016) processor; next is for Ubuntu 18.04.1
#       CCFLAGS = $(SDFLAGS) -fexceptions -fvisibility=hidden -m64 -mtune=generic -o $(INTDIR)/$@
        CXXFLAGS := -std=gnu++0x

        ifeq ($(TYPE),EXE)
          OUT_EXT := $(EXE_EXT)
          # LD := gcc
          LD := aarch64-poky-linux-ld
#         LDFLAGS = -Wl,--gc-sections -fno-keep-static-consts -fexceptions -march=armv8-a -maarch64linux -L$(OUTDIR) -o$@
# Use the above -Wl when using gcc. When using ld directly, it doesn't take -Wl. aarch64-poky-linux-ld doesn't have the -f options.
          LDFLAGS = --gc-sections -march=armv8-a -maarch64linux -L$(OUTDIR) -o$@
# That's for Linaro and the Snapdragon410 (Qualcomm APQ8016) processor; next is for Ubuntu 18.04.1
#         LDFLAGS = -Wl,--gc-sections -fno-keep-static-consts -fexceptions -m64 -mtune=x86-64 -L$(OUTDIR) -o$@
          LINUXSYSTEMLIBS := -lstdc++ -lrt -lpthread $(addprefix -l, $(LINUXSYSTEMLIBS))
        endif
        ifeq ($(TYPE),LIB)
          OUT_EXT := $(LIB_EXT)
          # LD := ar
          LDFLAGS = crus $@
        endif
        ifeq ($(TYPE),DLL)
          OUT_EXT := $(DLL_EXT)
          # LD := gcc
          LD := aarch64-poky-linux-ld
#         LDFLAGS = -Wl,--gc-sections -export-dynamic -fno-keep-static-consts -fexceptions -march=armv8-a -maarch64linux -L$(OUTDIR) -o$@
# Use the above -Wl when using gcc. When using ld directly, it doesn't take -Wl. aarch64-poky-linux-ld doesn't have the -f options.
          LDFLAGS = --gc-sections -export-dynamic -march=armv8-a -maarch64linux -L$(OUTDIR) -o$@
# That's for Linaro and the Snapdragon410 (Qualcomm APQ8016) processor; next is for Ubuntu 18.04.1
#         LDFLAGS = -Wl,--gc-sections -export-dynamic -fno-keep-static-consts -fexceptions -m64 -mtune=x86-64 -L$(OUTDIR) -o$@
          LINUXSYSTEMLIBS := -lstdc++ -lrt -lpthread $(addprefix -l, $(LINUXSYSTEMLIBS))
        endif
      endif

    endif #ifeq ($(PLATFORM),linuxApp)

  endif #ifneq ($(RELEASE),)
endif #ifneq ($(PLATFORM),)

################################################################################
# Phony target specification
################################################################################
.PHONY: goal all clean phonyclean $(NAME)\
        winntDebug winntRelease \
        winntDebug64 winntRelease64 \
        winntDebugClean winntReleaseClean \
        winntDebug64Clean winntRelease64Clean \
        linuxDebug linuxRelease \
        linuxDebugClean linuxReleaseClean

################################################################################
# Targets
################################################################################
goal: winntDebug winntRelease

all: winntDebug winntRelease \
     winntDebug64 winntRelease64

clean: winntDebugClean winntReleaseClean \
       winntDebug64Clean winntRelease64Clean

linuxRelease:
  ifneq (,$(findstring linux,$(TARGETS)))
    $(PREFIX)$(MAKE) -f $(NAME_MKE) $(NAME) --no-print-directory PLATFORM=linuxApp RELEASE=Release
  else
    $(PREFIX)echo $@ not configured for $(NAME)
  endif

linuxReleaseClean:
  ifneq (,$(findstring linux,$(TARGETS)))
    $(PREFIX)$(MAKE) -f $(NAME_MKE) phonyclean --no-print-directory PLATFORM=linuxApp RELEASE=Release CLEAN=Yes
  else
    $(PREFIX)echo linuxRelease not configured for $(NAME)
  endif

### Dependency files ###########################################################

$(INTDIR)/%.$(DEP_EXT) : %.c
    $(PREFIX)echo $(@F)
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$@),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$@),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$@.tmp),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$@.tmp),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(GINTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(GINTDIR)),$(IFNOTEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFNOTEXIST)))
  #
  # Do not use (OBJ_EXT) in the SED command, since all dependencies are generated with gcc.
  # Use .o instead
  #
    $(PREFIX)$(SD) $(SDCFLAGS) $(SDFLAGS) $< | $(SE) "s/$*.o:/$*.$(OBJ_EXT) $(subst /,\/,$@):/" > $(subst /,$(PATH_SEP),$@.tmp)
    $(PREFIX)$(subst _PATH_,$(subst /,$(PATH_SEP),$(INTDIR)/),$(subst _OLDNAME_,$(@F).tmp,$(subst _NEWNAME_,$(@F),$(RN))))

$(INTDIR)/%.$(DEP_EXT) : %.cpp
    $(PREFIX)echo $(@F)
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$@),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$@),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$@.tmp),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$@.tmp),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(GINTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(GINTDIR)),$(IFNOTEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFNOTEXIST)))
  #
  # Do not use (OBJ_EXT) in the SED command, since all dependencies are generated with gcc.
  # Use .o instead
  #
    $(PREFIX)$(SD) $(SDCPPFLAGS) $(SDFLAGS) $< | $(SE) "s/$*.o:/$*.$(OBJ_EXT) $(subst /,\/,$@):/" > $(subst /,$(PATH_SEP),$@.tmp)
    $(PREFIX)$(subst _PATH_,$(subst /,$(PATH_SEP),$(INTDIR)/),$(subst _OLDNAME_,$(@F).tmp,$(subst _NEWNAME_,$(@F),$(RN))))

### Object files ###############################################################

%.$(OBJ_EXT) : %.asm
    $(PREFIX)echo $(@F)
    $(PREFIX)$(AS) $(ASFLAGS) $<

%.$(OBJ_EXT) : %.S
    $(PREFIX)echo $(@F)
    $(PREFIX)$(AS) $(ASFLAGS) $<

%.$(RES_EXT) : %.rc
    $(PREFIX)echo $(@F)
    $(PREFIX)"$(RC)" $(RCFLAGS) -fo "$(INTDIR)/$(@F)" $<

%.$(OBJ_EXT) : %.c $(TOOL_DEPS)
  ifneq ($(CC),cl.exe)
    $(PREFIX)echo $(@F)
  endif
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)/$@),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$(INTDIR)/$@),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFNOTEXIST)))
    $(PREFIX)$(CC) $(CCFLAGS) $<

%.$(OBJ_EXT) : %.cpp $(TOOL_DEPS)
  ifneq ($(CC),cl.exe)
    $(PREFIX)echo $(@F)
  endif
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)/$@),$(subst _COMMAND_,$(RM) $(subst /,$(PATH_SEP),$(INTDIR)/$@),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFNOTEXIST)))
    $(PREFIX)$(CC) $(CCFLAGS) $(CXXFLAGS) $<

### Output file ################################################################

$(NAME): $(OUTFILE) $(BINFILE)

$(OUTFILE): $(INTDIR)/$(NAME).x $(OBJS) $(LIBS) $(NAME_MKE)
    $(PREFIX)echo $@
    $(PREFIX)$(subst _FILE_,$@,$(subst _COMMAND_,$(RM) $@,$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(OUTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(OUTDIR)),$(IFNOTEXIST)))
    ifneq (,$(findstring $(OBJINT),$(OBJS)))
        $(foreach obj,$(OBJS),$(shell echo $(obj) >> $(LSTFILE)))
    else
        $(foreach obj,$(OBJS),$(shell echo $(subst /,$(PATH_SEP),$(INTDIR)/$(obj)) >> $(LSTFILE)))
    endif
    ifneq (,$(filter $(PLATFORM),linuxApp))
      ifeq ($(TYPE),LIB)    
    $(PREFIX)ar $(LDFLAGS) @$(LSTFILE) $(LIBS) $(LINUXSYSTEMLIBS)
      else
        ifeq ($(TYPE),EXE)    
        $(PREFIX)aarch64-poky-linux-ld $(LDFLAGS) @$(LSTFILE) $(LIBS) $(LINUXSYSTEMLIBS)
        else
          ifeq ($(TYPE),DLL)    
        $(PREFIX)aarch64-poky-linux-ld $(LDFLAGS) @$(LSTFILE) $(LIBS) $(LINUXSYSTEMLIBS)
          else
        $(PREFIX)LD $(LDFLAGS) @$(LSTFILE) $(LIBS) $(LINUXSYSTEMLIBS)
          endif
        endif
      endif
    else
      ifeq ($(PLATFORM),winntApp)
        $(PREFIX)$(LD) $(LDFLAGS) @$(LSTFILE) $(WINNTSYSTEMLIBS)
      else
        $(PREFIX)$(LD) $(LDFLAGS) @$(LSTFILE)
      endif
    endif
    $(PREFIX)$(subst _FILE_,$(LSTFILE),$(subst _COMMAND_,$(RM) $(LSTFILE),$(IFEXIST)))

$(BINFILE): $(OUTFILE)
    $(PREFIX)echo $@
    $(PREFIX)$(B2B) $(B2BFLAGS) $(OUTFILE) $(BINFILE)

$(INTDIR)/$(NAME).x:
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(MD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFNOTEXIST)))
    $(PREFIX)echo Checkpoint file for component $(NAME) > $(subst /,$(PATH_SEP),$@)

### Clean ######################################################################

phonyclean:
  ifneq ($(PLATFORM),)
    ifneq ($(RELEASE),)
    $(PREFIX)$(subst _FILE_,$(subst /,$(PATH_SEP),$(INTDIR)),$(subst _COMMAND_,$(RD) $(subst /,$(PATH_SEP),$(INTDIR)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.pdb,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.pdb,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.map,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.map,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.dat,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.dat,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.exp,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.exp,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.lib,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.lib,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(patsubst %.$(OUT_EXT),%.err,$(OUTFILE)),$(subst _COMMAND_,$(RM) $(patsubst %.$(OUT_EXT),%.err,$(OUTFILE)),$(IFEXIST)))
    $(PREFIX)$(subst _FILE_,$(OUTFILE),$(subst _COMMAND_,$(RM) $(OUTFILE),$(IFEXIST)))
    else
    $(PREFIX)echo Target $@ should not be used from command line
    endif
  else
    $(PREFIX)echo Target $@ should not be used from command line
  endif

################################################################################
# Include dependency files
################################################################################
ifneq ($(PLATFORM),)
  ifneq ($(RELEASE),)
    #
    # If cleaning, we must not include dependency files.
    # Otherwise they will be created before cleaning.
    #
    ifeq ($(CLEAN),)
      ifeq (,$(findstring $(OBJINT),$(OBJS)))
        #
        # Use a - in front of the include, otherwise make will complain
        # when it is ran the first time (when there are no .dep files).
        #
        -include $(patsubst %.$(DEP_EXT),$(INTDIR)/%.$(DEP_EXT),$(DEPS))
      endif
    endif
  endif
endif

Could someone point me at a simple example of a complete Yocto layer including makefile and simple code that cross-compiles? It seems difficult for me to find, especially the makefile that supports being used in cross-compilation.

An important realization is where the documentation is about the path variables and other settings for the CFLAGS and LDFLAGS: with all variables in the Yocto mega-manual, in "Chapter 33. Variables Glossary", https://www.yoctoproject.org/docs/3.0/mega-manual/mega-manual.html#ref-variables-glos

Answer 1

Yocto dependency concepts

First, you seem to mix various dependency concepts in Yocto:

• IMAGE_INSTALL is an image dependency telling to Yocto the recipe in parameter must be installed in the final image. It should be placed inside image recipe and not inside package recipe. I don't think adding that into a package recipe tells to Yocto to add this dependencies to the recipe's staging directory.
• RDEPENDS is used inside package recipe and tells to Yocto the recipe depends on the packages in parameter at runtime. Then it will install in final image the required runtime dependencies if you install the package requiring them.
  A basic example is a recipe containing a script shell depends at runtime on some shell interpreter.
• DEPENDS is used also inside package recipe and tells to Yocto the recipe depends on the packages in parameters at compile time. It will add the header files and the libraries in the staging directory of your recipe.

That being said, libstdc++, libpthread and librt are part of the toolchain and you don't need to add explicit dependencies in your recipes for that. They will be installed in the staging directory or your recipe without any effort.

Regarding the libdns_sd, following my quick research, this library is provided by mDNS. You then need to add a dependency on it:

DEPENDS = "mdns"

As explained above, this will add the header files, *.a and *.so in the staging directory of your recipe.

What is missing ?

Once the dependencies are correctly configured, your compiler needs to know where are the include and library directories and this is clearly missing in your Makefile probably because you assume the dependencies are part of the host toolchain. When you build directly for the host, the compiler knows where to find includes (search in /usr/include for example) and libraries which makes this process quite simple.

However, as for a standard cross compilation project, in Yocto the headers and libraries are located elsewhere and you need to specify that correctly in your Makefile.

There is multiple possibilities to manage a Makefile project in Yocto but in all cases, you need a Makefile which can receive variables (CFLAGS, LDFLAGS, etc) or optional arguments to pass to the compiler/linker when called from your recipe.

Since you don't give the Yocto recipe, I will give an example on how it can be done and some suggestions. Also, I focus on compile aspect, the example is not complete and should probably be adapted for other build steps (like install).

DEPENDS += "my-extralib"

# Change eventually the default source directory
S = "${WORKDIR}/git"

EXTRA_OEMAKE = " 'CC=${CC}' \
    'CFLAGS=${CFLAGS} ${TARGET_CC_ARCH} -I${S}/inc -I${STAGING_DIR_TARGET}${includedir}/my-extralib-include' \
    'LDFLAGS=-lrt -lpthread -lm -lmy-extralib -L${STAGING_DIR_TARGET}${libdir}/my-extralib-libdir' \
    'BUILDDIR=${S}' \
    'OTHER_DEFINE=${OTHER_YOCTO_VAR}' \
"

Using EXTRA_OEMAKE lets Yocto do a lot of tasks automatically for you like the configure, compile and install. If you want to override them, this is also possible:

do_compile() {
    # modify the Makefile on the fly as an example
    cp ${S}/Makefile.orig ${S}/Makefile
    sed -i -e 's:= $(LDFLAGS):?= $(LDFLAGS):' ${S}/Makefile
    oe_runmake SOME_OVERRIDE='some-override'
}

You can also disable a step like the configure which is probably not used in your case:

do_configure[noexec] = "1"

Some tips

• If you start with Yocto, there is a lot to learn and it can be difficult to find how to manage all the possibilities and options at the beginning. I strongly suggest you start with a simple Makefile and minimum source files to keep focus on the Yocto side.
• You can find a lot of recipe examples on the web (also Yocto, Poky base recipes) which are probably close to you are trying to achieve. The documentation of Yocto is really complete but it is more a base reference. Concrete implementations can be found in existing recipes.
• The temp directory inside the build directory of your recipe is a gold mine with all the scripts and logs of the different building steps. This is the first place to look when you are creating your recipe.