gtk+: always use our cairo

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

Formula/gtk+.rb

@@ -23,6 +23,11 @@ class Gtkx < Formula
   end
 
   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}",