Genex: Teach LANG_COMPILER_ID and friends to support multiple ids
Allow $<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>
to be written as $<CXX_COMPILER_ID:GNU,Clang,AppleClang>
.
Allow $<OR:$<CXX_COMPILER_ID:GNU>,$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>
to be written as $<CXX_COMPILER_ID:GNU,Clang,AppleClang>
.