gtk+3: always use our cairo

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/gtk+3.rb

@@ -13,11 +13,16 @@ class Gtkx3 < Formula
   depends_on 'libtiff'
   depends_on 'gdk-pixbuf'
   depends_on 'pango'
-  depends_on 'cairo' # for cairo-gobject; XQuartz includes it, but it's broken as of 2.7.2
+  depends_on 'cairo'
   depends_on 'jasper' => :optional
   depends_on 'atk' => :optional
 
   def install
+    # Always prefer our cairo over XQuartz cairo
+    cairo = Formula.factory('cairo')
+    ENV['CAIRO_BACKEND_CFLAGS'] = "-I#{cairo.include}/cairo"
+    ENV['CAIRO_BACKEND_LIBS'] = "-L#{cairo.lib} -lcairo"
+
     system "./configure", "--disable-debug",
                           "--disable-dependency-tracking",
                           "--prefix=#{prefix}",