[CMake] adding file extensions for languages

[Justin Cook]

Hello,

I was trying to add some file extensions for the Fortran language (.ftn), but I was not able to get past some errors even after trying a few different options including appending to CMAKE_Fortran_SOURCE_FILE_EXTENSIONS in a toolchain file.

Ideas on how to get around the following error(s)? It looks like set_target_property might work, but I'd rather just add the extension to work as any other accepted extension.

-- Configuring done
CMake Error: CMake can not determine linker language for target: ftn_mpi_hello
CMake Error: Cannot determine link language for target "ftn_mpi_hello".
-- Generating done

Thanks,

Justin