png: Allow building shared freetype with bundled libpng

This was the behaviour when building Godot 2.1, which allows to build against
Ubuntu 12.04 and its freetype that links old libpng12, while still bundling
libpng16.

(cherry picked from commit 4965ddfaa1536fb179d4b156d1ecb8a72a3c565f)

platform/server/detect.py

@@ -76,7 +76,6 @@ def configure(env):
         env.ParseConfig('pkg-config libwebp --cflags --libs')
 
     if (env['builtin_freetype'] == 'no'):
-        env['builtin_libpng'] = 'no'  # Freetype links against libpng
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):

platform/x11/detect.py

@@ -140,7 +140,6 @@ def configure(env):
         env.ParseConfig('pkg-config libwebp --cflags --libs')
 
     if (env['builtin_freetype'] == 'no'):
-        env['builtin_libpng'] = 'no'  # Freetype links against libpng
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):