Fix a few issues with building on OSX

NOTE: The version of OpenGL shipping with OSX only really supports the
OGL2 examples for now. I haven't been able to completely figure out why
the OGL3 examples just die.

There were two separate issues fixed here. Firstly, it allows FreeGlut to be found when using a version installed from homebrew and secondly, it fixes a few import errors related to OSX shipping OpenGL with the OpenGL include prefix and not GL, like everyone else.