The verification program entry point (main) is defined in a C source
file, so the C compiler should be used to link when only Fortran and C
are involved. The C++ compiler should still be used when the CXX option
is enabled.
This function builds a simple test project using a combination of
Fortran and C (and optionally C++) to verify that the compilers are
compatible. The idea is to help projects report very early to users
that the compilers specified cannot mix languages.