CCUserDefault.cpp: Should not compile on Mac platform

On the Mac, the CCUserDefault.mm implementation should be used (it already checks for the Mac platform). That way NSUserDefaults can be used.

On a related note, why is KEEP_COMPATABILITY defined at the top of CCUserDefault.mm? Will this be removed in future releases? Otherwise, the only way to use NSUserDefaults is to remove the define. Should this setting be moved to Configuration or ccConfig?