Use OpenMP include path
This is needed when the OpenMP header is not in the default search path. On macOS with libomp installed from MacPorts, the following now works:
cmake . \
-DCMAKE_BUILD_TYPE=Release \
-DWALBERLA_BUILD_WITH_MPI=OFF \
-DWALBERLA_BUILD_WITH_OPENMP=ON \
-DOpenMP_libomp_LIBRARY=/opt/local/lib/libomp/libomp.dylib \
-DOpenMP_C_INCLUDE_DIR=/opt/local/include/libomp \
-DOpenMP_CXX_INCLUDE_DIR=/opt/local/include/libomp