Fix OSX build of glsl_test

Without this patch, OSX builds were failing at link, unable to find the OpenGL libraries:

{{{
Linking CXX executable glsl_test
Undefined symbols for architecture x86_64:
  "_CGLChoosePixelFormat", referenced from:
      _main in glsl_optimizer_tests.cpp.o
  "_CGLCreateContext", referenced from:
      _main in glsl_optimizer_tests.cpp.o
....
}}}