CMake doesn't link C and C++ static libraries (undefined reference to function)

The problem here is, that the linker relies on the order of the libraries. With

target_link_libraries(prog funcc_lib funccpp_lib)

It first links funcc_lib and then funccpp_lib. But it never gets back to funcc_lib. Since funccpp_lib depends on funcc_lib, you have to change the order of the libraries:

target_link_libraries(prog funccpp_lib funcc_lib)

For additional information, see this discussion.