CMake 3.0 compatibility: use AppleClang for Clang on OS X.
CMake 3.0 compatibility: use AppleClang for Clang on OS X.

Different name because of different version numbers. Also we need to
stay compatible with <3.0, which defines it as "Clang" only.

DerThorsten/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,ashimidashajia/magnum,MiUishadow/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum