Make some changes to dependencies on Mac OS.

 - when using quartz, we don't need fontconfig, freetype or harfbuzz

 - similarly, we won't need at-spi either

 - cairo GL support is currently broken on Mac OS, so disable it.
   See https://bugs.freedesktop.org/show_bug.cgi?id=87707

 - rework Gtk to enable backends (including quartz) according to
   condition flags

 - libtiff support is disabled in gdk-pixbuf just because I don't think
   that it's really important and it removes one extra dependency to
   bootstrap.  Ideally, we'd use quartz image loaders instead.
   See https://bugzilla.gnome.org/show_bug.cgi?id=720762