Added libomp formula for 64 bit OS X.

libomp is the OpenMP Runtime Library by Intel. libomp dylib and headers are needed in Xcode projects to write OpenMP programs using clang.
The syntax is confusing due to History - sorry about that. `fails_with :gcc` means GCC 4.2, while `fails_with :gcc => "number"` refers to other numbered series. So `fails_with :gcc => "4.3"` means the whole GCC 4.3 series, `fails_with :gcc => "4.8"` means GCC 4.8, and so on.