New formula: gtk+3

A demonstration of the new X11/XQuartz dependency support. Includes
relevant fixups for compatibility.

Closes Homebrew/homebrew#12552.

Formula/gtk+3.rb

@@ -0,0 +1,32 @@
+require 'formula'
+
+class Gtkx3 < Formula
+  homepage 'http://gtk.org/'
+  url 'http://ftp.gnome.org/pub/gnome/sources/gtk+/3.4/gtk+-3.4.3.tar.xz'
+  sha256 '846010442a96590469cde16ed6a1fdfe09397e435165459f04d31cfa5713799f'
+
+  depends_on :x11 => '2.5' # needs XInput2, introduced in libXi 1.3
+  depends_on 'pkg-config' => :build
+  depends_on 'xz' => :build
+  depends_on 'glib'
+  depends_on 'jpeg'
+  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 'jasper' => :optional
+  depends_on 'atk' => :optional
+
+  def install
+    system "./configure", "--disable-debug",
+                          "--disable-dependency-tracking",
+                          "--prefix=#{prefix}",
+                          "--disable-glibtest",
+                          "--disable-introspection"
+    system "make install"
+  end
+
+  def test
+    system "#{bin}/gtk-demo"
+  end
+end