glx: Fix GLX_CONTEXT_RELEASE_BEHAVIOR_ARB handling
None of this was getting compiled because we hadn't defined the macro (and aren't getting them from <GL/glxext.h> because reasons). Fix that. Fixes: xorg/xserver#684
None of this was getting compiled because we hadn't defined the macro (and aren't getting them from <GL/glxext.h> because reasons). Fix that. Fixes: xorg/xserver#684