Log Message:
Several changes:

* It's CONFIGURE_ARGS, not CONFIGURE_ARG.
* Use Mesa/buildlink.mk to properly handle whether we're using XF3 or XF4
  (the latter contains its own MesaGL libraries).
* Comment out inclusion of x11.buildlink.mk since we're not even weakly
  buildlinked, so jumping ahead to trying to strongly buldlinkify this
  package is overboard.
* Sort the buildlink.mk file (just personal aesthetics).