OSX 10.9 Mavericks target deployment

#652 

This allows 10.9 to be a CMAKE_OSX_DEPLOYMENT_TARGET. The build issue is due to 10.9 using libc++ by default which causes incompatibility issues with libs compiled against libstdc++ (default in <10.9). This pull request explicitly sets the stdlib=libstdc++ for all Apple builds. 

I have tested on 10.9 and 10.8 machines and both build, pass (model) tests, and pass manual IRB ruby script testing.

This should only affect Mac builds although @axelstudios please make sure it doesn't affect Linux or Window builds.

@axelstudios @lefticus @kbenne