Drop PPC support
SDL 2.0.2 will completely drop the PPC support, so do it on Solarus too.
CMake : Allow universal (x86+x64) build with the XCode generator.
According to http://public.kitware.com/Bug/view.php?id=14733 , the XCode generator is the only one to allow to set arch-specific flags for now.
Merge branch 'master' of https://github.com/christopho/solarus