FindOpenMP: Handle non-Apple clang on macOS

I was noticing that FindOpenMP was failing to find my openmp installation
on macos when building with a custom-compiled clang. Upon inspection it
appears that FindOpenMP is looking for `AppleClang` as a compiler
identification and the `Clang` case is only applicable for Win32.
Try to fix that by treating custom `Clang` on macos the same as
`AppleClang`.