Better Clang-CL support
- clang-cl currently does not recognize (MSVC's) /std:c+latest - consider generating proper Clang flags instead (for CMAKE_CXX_STANDARD)
- when targeting clang-cl generator expressions seem not to work properly with targets that mix .c and .cpp sources: using something like $<$<COMPILE_LANGUAGE:CXX>:/clang:-std=gnu++2b> fails because the flags gets set on C sources as well (and clang-cl errors out on that saying it is an unsupported flag for C sources)