Proff Of Concept for CMAKE_VERIFY_INTERFACE_HEADER_SETS

CMAKE_VERIFY_INTERFACE_HEADER_SETS checks that all public header FILE_SETS are compilable on their own.

Here I have modified the code so that it compiles but kar.h is not compilable on its own and it's being correctly caught.

Unfortunately means having some ugly ifs in the code, not sure if it's worth it