povray: use internal libpng

povray won't compile against libpng1.5; this patch to the configure
script causes it to use its internal libpng, which works.

(I attempted to use the Macports patches to fix it for 1.5, without
much success. If anyone else wants to take a look, please do!)

Fixes Homebrew/homebrew#14895.

Formula/povray.rb

@@ -13,6 +13,13 @@ class Povray < Formula
     cause "povray fails with 'terminate called after throwing an instance of int'"
   end if MacOS.version == :leopard
 
+  def patches
+    # povray has issues determining libpng version; can't get it to compile
+    # against system libpng, but it works with its internal libpng.
+    # Look at this again on the next povray version bump!
+    {:p0 => "https://trac.macports.org/export/97719/trunk/dports/graphics/povray/files/patch-configure"}
+  end
+
   def install
     system "./configure", "--disable-debug", "--disable-dependency-tracking",
                           "COMPILED_BY=homebrew",