libpng: convert recipe to actual recipe

media-libs/libpng/patches/libpng-1.5.12.patchset

@@ -0,0 +1,22 @@
+From ca47ea510b8f381f4cd3f0dbf887faa0b95257ca Mon Sep 17 00:00:00 2001
+From: Ingo Weinhold <ingo_weinhold@gmx.de>
+Date: Mon, 10 Jun 2013 23:42:11 +0200
+Subject: configure.ac: replace obsolete AM_CONFIG_HEADER
+
+
+diff --git a/configure.ac b/configure.ac
+index beeeb81..8f51ada 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -31,7 +31,7 @@ PNGLIB_RELEASE=12
+ dnl End of version number stuff
+ 
+ AC_CONFIG_SRCDIR([pngget.c])
+-AM_CONFIG_HEADER(config.h)
++AC_CONFIG_HEADERS(config.h)
+ 
+ # Checks for programs.
+ AC_LANG([C])
+-- 
+1.7.5
+