CMake: Add appropriate c++11 flag for clang.
CMake: Add appropriate c++11 flag for clang.

pizcogirl/Astron,ketoo/Astron,blindsighttf2/Astron,pizcogirl/Astron,ketoo/Astron,pizcogirl/Astron,blindsighttf2/Astron,blindsighttf2/Astron,pizcogirl/Astron,ketoo/Astron,blindsighttf2/Astron,ketoo/Astron