Fix mac osx compiling (Mac OS X 10.8)

I was unable to compile and link sfgui on my mac running OS X 10.8 against SFML 2.1.

I got it working, which basically meant to add a check in the CMakeList for setting the -std++ flag for clang and making sure linking was done correctly (not only core foundation).

On top of that I suppressed warnings which occurred quite a lot and updated the readme for mac users.