CMakeLists.txt: enable c++11

otherwise we se many errors like:

| git/src/compositor/shell.h:66:10: error: 'void Shell::shell_bind_resource(QtWaylandServer::hawaii_shell::Resource*)' marked override, but does not override
     void shell_bind_resource(Resource *resource) Q_DECL_OVERRIDE;

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>