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.

(cherry picked from commit c32766a482595256bc48155587a47f27848ac8ea)

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')
 
     # Sound and video libraries
     # Keep the order as it triggers chained dependencies (ogg needed by others, etc.)

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_squish'] == 'no' and env["tools"] == "yes"):
         env.ParseConfig('pkg-config libsquish --cflags --libs')