setting up cmake to build and link pybind11

Question

I have a C++ project with a folder structure like this:

src/
    CMakelists.txt
    pynative/
        CMakelists.txt
        include/
        pynative.cpp        
    include/
       foo.h
       // more headers
    lib.cpp
    // more source files

The pynative.cpp file has a PYBIND11_MODULE definition to export some functions which use code from the parent src/include/. I want to compile and export the pybind11 module with all the required source code from src/include/ and lib.cpp. Here's my src/CMakelists.txt:

cmake_minimum_required(VERSION 3.5.1)

set(CMAKE_CXX_COMPILER "CLANG")
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

project(mainproj VERSION 0.1.0 LANGUAGES CXX)

find_package(LLVM REQUIRED CONFIG)
include_directories(include)
add_library(mainproj lib.cpp)
add_subdirectory(pynative)
target_link_libraries(pynative PUBLIC mainproj)

And here's my pynative/CMakelists.txt:

cmake_minimum_required(VERSION 3.5.1)

project(pynative VERSION 0.1.0 LANGUAGES CXX)

list(APPEND CMAKE_PREFIX_PATH C:/Users/somepath/)

find_package(pybind11 REQUIRED)
pybind11_add_module(pynative pynative.cpp)

This is the first time I'm working with Cmake and its hard to understand why I'm getting this error:

  Attempt to add link library "mainproj" to target "pynative"
  which is not built in this directory.

  This is allowed only when policy CMP0079 is set to NEW

The mainproj is just a set of functions no main so I don't want to compile it to an executable, I want it as a library which links with pynative to give me a Python module I can use in my code. I know mainproj is being built outside of pynative but not sure what can I do to fix this. Please help.

Also, is my current project structure a good way to organize this type of code? Should I move mainproj to a sibling directory?