libpng: Fix erroneously linking against libpng12 on old distros

This bit us for 2.1.1 binaries built on Ubuntu 12.04 LTS where
libpng.pc apparently prioritizes libpng12.

platform/x11/detect.py

@@ -144,7 +144,7 @@ def configure(env):
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):
-        env.ParseConfig('pkg-config libpng --cflags --libs')
+        env.ParseConfig('pkg-config libpng16 --cflags --libs')
 
     if (env['builtin_enet'] == 'no'):
         env.ParseConfig('pkg-config libenet --cflags --libs')