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.
I misunderstood the compatibility table. GCC isn't supported at all. The 4.6.2 footnote applies for Linux only. So to generally fail for all GCC versions this should be sufficient, right?
```ruby
fails_with :gcc do
  cause "libomp can only be built with clang."
end
```