Scanning the ECM for something that could explain a symbol visibility issue I'm seeing with AppleClang but not with Clang I noticed that this compiler isn't always detected the same way.
This patch ensures that CMAKE_<LANG>_COMPILER_ID MATCHES "Clang" is used throughout, i.e. MATCHES instead of STREQUAL.
This includes 1 or 2 cases where AppleClang is unlikely to be the compiler in use (for instance because of AND NOT APPLE) but I think it's better to use the same detection principle everywhere if not only to prevent inappropriate copy/paste edits in the future.