Fix #20379: Anttweakbar fails on OS X 10.8

This is a patch to the anttweakbar formula to address 

https://github.com/mxcl/homebrew/issues/20379
https://sourceforge.net/p/anttweakbar/tickets/8/

Tested on OS X 10.8.4

This fix makes two copies of OpenGL function signatures included within the code consistent with the system headers and the signatures posted in the official OpenGL registry.