Log Message:
Re-enable glx on OS X.

Upstream disabled glx on OS X (in an apparent attempt to make it build
on systems without X11).  However, libepoxy without glx is defective,
and gtk3 fails with it.  So it seems like the right thing is to ensure
X11 presence for libepoxy.

This patch reverts upstream's disabling of glx (setting configure
variables/defines), making it be like libepoxy 1.2.  The resulting
libepoxy has glx, and gtk3+ builds fine against it on OS X 10.9.

(It remains TBD to sort out how we feel about X vs quartz, but that's
a separable issue.)

Reviewed by ttn@ and tron@.