FindOpenMP: Use OpenMP_<lang>_INCLUDE_DIR
If OpenMP_<lang>_INCLUDE_DIR
is defined, add it to the list of include
directories before checking flags. Previously, this variable was
ignored for all compilers but AppleClang, despite the documentation
mentioning it as one of the possible inputs.
Fixes: #24260 (closed)
Topic-rename: FindOpenMP-test-include-dir
Edited by Brad King