leptonica: add PNG support

leptonica doesn't look for libpng in /usr/X11/lib, so it was compiling
without PNG support. Also add a patch to fix a bug in PNG support in
leptonica 1.68, which will be fixed in the next version.
Closes Homebrew/homebrew#8212.

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

Formula/leptonica.rb

@@ -8,7 +8,15 @@ class Leptonica < Formula
   depends_on 'jpeg'
   depends_on 'libtiff'
 
+  def patches
+    # Leptonica is missing an #include for PNG support
+    # Can be removed in 1.69
+    # http://code.google.com/p/leptonica/issues/detail?id=56
+    "http://leptonica.googlecode.com/issues/attachment?aid=560001000&name=zlib-include.patch&token=c41d407131e91bb18eb4f3d2d569c778"
+  end
+
   def install
+    ENV.x11
     system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
     system "make install"
   end