FindOpenMP: Add support for AppleClang

This patch fixes three things:

1. CMAKE_ROOT is used in both include statement, rather than being mixed with CMAKE_CURRENT_LIST_DIR.
2. Actually add an OpenMP function to make sure we can link to OpenMP library (like the Fortran example).
3. Add a search for the correct lib when compiling with AppleClang. I've used a very similar procedure to the one used for Intel.

Fixes: #17775 (closed)
Topic-rename: FindOpenMP-AppleClang