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.
Okay, this will need some `fails_with` statements so that it prevents it from being built with GCC:

```ruby
fails_with :gcc
fails_with :gcc_4_0
(4.3..4.9).each do |n|
  fails_with :gcc => n
end
```

With a comment at the top explaining why.