They're needed on MacOS as SDL_opengl_gletx.h doesn't define them there.
They don't actually work on MacOS, so long-term, the code that uses
these defines should be changed to use #ifdef to check they're available
before using them.
As well as fixing the MacOS build, it meddled with a bunch of files that
already did the right thing.
This reverts commit 8162541a14, reversing
changes made to e16c451d08.