add libomp  library dependency with OpenMP and Clang
add libomp  library dependency with OpenMP and Clang

`clang++-mp-X.Y -fopenmp ...` links against libraries in libomp.

neverpanic/macports-base,danchr/macports-base,neverpanic/macports-base,macports/macports-base,danchr/macports-base,macports/macports-base