Fix OpenMP detection on macOS

[virtualritz]

This broken in the resp. CMake module. I.e. even when libomp and/or clang are installed via Homebrew and even when the Homebrew Clang is set as the compiler, OpenMP is not properly detected.

https://stackoverflow.com/questions/46414660/macos-cmake-and-openmp https://stackoverflow.com/questions/48825416/missing-openmp-c-flags-openmp-c-lib-names

https://gitlab.kitware.com/cmake/cmake/-/issues/21787