downgrade to libpng-1.5.18 to avoid issues related to the sRGB profile

Some changes in libpng 1.6+ cause it issue warning or even not work
correctly with the original HP/MS sRGB profile. The message you may get
is as follows (originally reported on #1763):

libpng warning: iCCP: Not recognizing known sRGB profile that has been edited

It may result in the profile getting removed, as per
http://sourceforge.net/p/png-mng/mailman/message/32029459/

scripts/build.py

@@ -291,8 +291,8 @@ DEPENDENT_LIBS = {
 
     'libpng': {
         'order' : 3,
-        'url' : 'http://downloads.sourceforge.net/libpng/libpng-1.6.10.tar.gz',
-        'sha1': 'cf81cf7df631bbfa649600b9a45d966b6bccac25',
+        'url' : 'http://downloads.sourceforge.net/libpng/libpng-1.5.18.tar.gz',
+        'sha1': '7ddf6865aa70d2d79faf65ebc611919a0b573654',
         'build' : {
             'msvc*': {
                 'result': {