Add missing flag to OS X clang flag (for 3.11 libv8)

I had some [issues](https://github.com/cowboyd/therubyracer/issues/301#issuecomment-60460635) with libv8 3.11 after upgrade to OS X 10.10, and after some investigation I have fixed it by applying [this](https://github.com/cowboyd/libv8/pull/124) patch and installing libv8 manually.

Here is same patch but for 3.11 branch, hope this will help someone!