Oops, forgot the libwxpng case.

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@7184 8ced0084-cf51-0410-be5f-012b33b47a6e

Externals/libpng/SConscript

@@ -6,6 +6,9 @@ import sys
 if env.has_key('shared_png') and env['shared_png']:
 	Return()
 
+if env['HAVE_WX']:
+	Return()	# Currently using libwxpng
+
 files = [
 	'png.c',
 	'pngerror.c',