Added in support for a couple of OSX specific glut constants

Hi

I hope this is okay. Not really done a change to a gem before so I'm not sure if I've done this right or have followed the correct protocol so apologies in advance for any faux pays. I've probably got the versioning stuff all wrong

I've only tested this on the mac but can't see any reason this would cause a problem on any other platforms.

Thanks in advance

gaz