patched makefile to use libc++ instead of libstdc++ on OSX

Patched Makefile to use libc++ instead of libstdc++ on OSX. This is done to enable C++11 features.

I'm not sure if this will cause issues or not since a lot of the libraries we might be using might be built with libstdc++.
But yeah this works on my machine. It'd be cool if someone could make sure it doesn't break Windows/Linux builds.