CheckIncludeFiles: Add LANGUAGE CXX option

David Adamrequested to merge
zanchey/cmake:CheckIncludeFilesCXX into master

Ported from CheckIncludeFiles.cmake

I wasn't sure whether to add any tests or update the documentation in CheckIncludeFile{,CXX,s}.

Topic-rename: CheckIncludeFiles-language-CXX

Edited by Brad King

Merge request reports