Update libpng to 1.2.38 to patch a minor security issue.
diff --git a/scripts/libpng-config.in b/scripts/libpng-config.in
index 2f475be..7ae7d50 100755
--- a/scripts/libpng-config.in
+++ b/scripts/libpng-config.in
@@ -4,7 +4,10 @@
# provides configuration info for libpng.
# Copyright (C) 2002, 2004, 2006, 2007 Glenn Randers-Pehrson
-# For conditions of distribution and use, see copyright notice in png.h
+
+# This code is released under the libpng license.
+# For conditions of distribution and use, see the disclaimer
+# and license in png.h
# Modeled after libxml-config.