C++ modules support?

The C++ modules standard is slated for 2020, but the basic implementation is currently available in modern compilers. CMake should begin to implement the ordered compilation of C++ files with module statements (similar to how fortran modules are handled), should support the current nonstandard compiler options (which can later be adjusted to the "official" flags with a minor commit), and should provide appropriate options for handling C++ modules.

CMake can help developers to remove C++ headers TODAY! CMake can once and for all ease the pain of C++ headers!