C++ Modules: Add source file property to marek a source as a module
requested to merge jessesna/cmake:msvc_set_source_files_properties_LANGUAGE_CompileAsCppModule into master
Create a new LANGUAGE "CXXMODULE" for set_source_files_properties PROPERTIES.
It enables a MSBuild value of "CompileAsCppModule", allowing to explicitly mark files as modules if their suffix doesn't match with the VS standard (ixx). This is only handled in Visual Studio at the moment. Example use: set_source_files_properties(${SOURCE} PROPERTIES LANGUAGE CXXMODULE)
Edited by Brad King