Unable to generate executable file using compiler

Question

I am trying to compile my source code using riscv64-unknown-elf-g++ compiler into a binary file but I am getting this warning and unable to generate exec file:

warning for library: libofdm.a the table of contents is empty (no object file members in the library define global symbols)

referring to this issue before:

Can't make my c++ file into a library with compiler

as suggested in the above answer the same toolchain is used to compile the object file to create the library. Even though I am getting the error. Below I am attaching the whole command line used to compile.

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(base) k:~ cd /Users/yuli/Documents/version3/cpp/
(base) k:~ ls
CMakeLists.txt  src     test
(base) k:~ mkdir build_riscv
(base) k:~ cd build_riscv
(base) k:~ cmake ..
-- The C compiler identification is AppleClang 11.0.3.11030032
-- The CXX compiler identification is AppleClang 11.0.3.11030032
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Compiling RISCV
CMAKE_CXX_COMPILER: /usr/local/opt/riscv-gnu-toolchain/bin/riscv64-unknown-elf-g++
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/yuli/Documents/version3/cpp/build_riscv
(base) k:~ make
Scanning dependencies of target ofdm
[ 33%] Building CXX object CMakeFiles/ofdm.dir/src/transmitter.cpp.o
[ 66%] Building CXX object CMakeFiles/ofdm.dir/src/configuration.cpp.o
[100%] Linking CXX static library libofdm.a
warning: /Library/Developer/CommandLineTools/usr/bin/ranlib: warning for library: libofdm.a the table of contents is empty (no object file members in the library define global symbols)
[100%] Built target ofdm
(base) k:~ 

and FYI I am using mac machine. I am Specifying this because I saw in a post with "clang warnings macOS #3331".

clang warnings macOS #3331

CMakeLists.txt as follows:

cmake_minimum_required(VERSION 3.13)
project(ofdmchain)

set(CMAKE_CXX_COMPILER "/usr/local/opt/riscv-gnu-toolchain/bin/riscv64-unknown-elf-g++")

add_definitions(-DCOMPILES_ON_PC -Wall -Wextra)

configure_file(${CMAKE_SOURCE_DIR}/src/config.hpp.in ${CMAKE_BINARY_DIR}/config.hpp)

if("${CMAKE_CXX_COMPILER}" MATCHES ".*riscv64-unknown-elf-g\\+\\+")
  message(STATUS "Compiling RISCV")
  SET(RISCV 1)
else()
  message(STATUS "Compiling X86")
  SET(RISCV 0)
endif()

message("CMAKE_CXX_COMPILER: ${CMAKE_CXX_COMPILER}")

add_library(ofdm
  src/transmitter.cpp
  src/configuration.cpp

  src/ofdm.hpp
  src/datatypes.hpp
  )
target_include_directories(ofdm PRIVATE ${CMAKE_SOURCE_DIR}/../reference_matlab)
target_include_directories(ofdm PRIVATE ${CMAKE_SOURCE_DIR}/src/)

if (NOT ${RISCV})
  add_executable(unit_tests
    test/catch_main.cpp
    test/test_sanity.cpp
    test/test_utilities.cpp
    test/test_transmitter.cpp
    )
  target_include_directories(unit_tests PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
  target_include_directories(unit_tests PUBLIC ${CMAKE_SOURCE_DIR}/src)
  target_link_libraries(unit_tests ofdm)
endif()