Alpaka: Add OMP to OSX CI tests

[BenjaminW3]

[ax3l]

Just a note:

It would be good if the OSX tests can compile with clang as well.
AppleClang is the default compiler (with XCode) on OSX and by default it builds with an older libc++.

[BenjaminW3]

TBB added with #742 #746

[BenjaminW3]

Without a real mac it is not easy to get the OpenMP backends working.
Installing it via brew install libomp (next to brew install tbb) works but then the build fails.
Coud someone with a mac try to get this working? @ax3l?

[ax3l]

I also just have an old mac at home that is not really my own :D
Can you link the branch and CI build you currently work on?

[BenjaminW3]

It is the topic-osx-omp branch on my fork. You probably have to revert the changes I did to the alpakaConfig.cmake file.

[q-p]

I was using a homebrew-installed gcc@7 (or 8) successfully on a Mac to compile alpaka-using software.

Note that the system compiler (Apple-customized clang) unfortunately doesn't include OpenMP support. :(

Note that libgomp (gcc's OpenMP support library) is really quite slow on Mac, same as anything else that build pthreads on the system (e.g. std::mutex, etc.). You can link against Intel's OpenMP runtime (libiomp IIRC) for a decent speed-up, as that provides libgomp-compatible symbols. The Intel OpenMP runtime can be used from homebrew's llvm / clang (by manually linking against it instead of using -fopenmp), for example.