Mac OS X Compilation fix

It took me a while, but in the end I managed to compile GtkRadiant on Mac OS X 10.8. For some reason SCons has hardcoded path to 10.4 SDK, so I added a fallback to pkg-config if that SDK is not found.

Zlib and GLU was not linked on my system (although it was linked on my linux machine), so I added that as well.

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