[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
Yes - i copied that code from stackoverflow. I remember that NSAppKitVersionNumber is a bit tricky (thats why i don't use it for detecting mavericks and later). I think this code also would detect snow leopard beta versions as snow leopard (which would be correct). In real snowleopard is 10.6.x (0 <= x <= 8) but i guess NSAppKitVersionNumber is also increased on pre-release versions ...