Revert "libpng: Fix erroneously linking against libpng12 on old distros"

This reverts commits 5fa1bb331ad9be31dbfc752c7d19ccf7caeb8fa
and ec4be71fade1ee5c6171e323d09197f3bf528499.

Looks like Debian/Ubuntu are not even shipping libpng16 nowadays in their
stable releases, we'll have to go back to statically linking our own
libpng16 to wait for them to stop being 5 years behind everybody.

platform/server/detect.py

@@ -80,7 +80,7 @@ def configure(env):
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):
-        env.ParseConfig('pkg-config libpng16 --cflags --libs')
+        env.ParseConfig('pkg-config libpng --cflags --libs')
 
     if (env['builtin_enet'] == 'no'):
         env.ParseConfig('pkg-config libenet --cflags --libs')

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 libpng16 --cflags --libs')
+        env.ParseConfig('pkg-config libpng --cflags --libs')
 
     if (env['builtin_enet'] == 'no'):
         env.ParseConfig('pkg-config libenet --cflags --libs')