[osx] - only show the fake fullscreen option on snow leopard

On later OSX versions this setting leads to black screen (deep core sdl issue we don't wanna fix ...). There are some users already who have locked theirselfs out because of this.

Please consider it for Gotham.

@Montellese - i think i got it - but i just wanna ping someone about it :D
Ah yes, the beta versions, that makes sense.