Add -lm to -lGL/-lMesaGL tests, to ensure it doesn't fail to find GL
just because libGL/libGLU use sqrt etc.!
Discard tests which search for IrisGL and PEX, since we can't use them anyway.
Re-install "depended"/"Makedepend" fix.  We really do want to force
Makedepend to be rebuilt, I think.