glib2-devel: Default to +quartz when -x11 is given

When building pango -x11 in a clean prefix, glib2 will fail to install,
because the -x11 variant is being passed down to glib2, but glib2
requires either +quartz or +x11, and +quartz was not enabled by default
when x11 was disabled.

This caused problems when implementing variant support for the buildbot
in macports/mpbb#5 and initially caused me to
think that I would have to revert
  macports/mpbb@f6e4681
which was added due to macports/mpbb#4 and
https://lists.macports.org/pipermail/macports-dev/2017-June/035978.html.

This solution should instead work without the revert and still allow the
buildbot to build both wine and +quartz-x11 ports.

See: https://trac.macports.org/ticket/52742

devel/glib2-devel/Portfile

@@ -134,7 +134,9 @@ variant x11 conflicts quartz {
 if {![variant_isset quartz]} {
     default_variants +x11
 }
-
+if {![variant_isset x11]} {
+    default_variants +quartz
+}
 if {![variant_isset quartz] && ![variant_isset x11]} {
     pre-configure {
         return -code error "Either +x11 or +quartz is required"