Right now it's simply broken of course. The extension being defined in the header is no guarantee the driver actually supports it.
That's the reason for the patch.
Of course my patch doesn't fix the fact that we just assume this extension is supported. For perfection we'll need a codepath that is based on OpenGL 1.0 texture combiners. I guess I'll do that as a fun project once I can get my hands on this old Mach64 card again...