Instead of defining new symbols for the windowing systems enabled at
configure time, we can reuse the same symbols for both the compile time
and run time checks, e.g.:

  #ifdef CLUTTER_WINDOWING_X11
    if (clutter_check_windowing_backend (CLUTTER_WINDOWING_X11))
      /* use the clutter_x11_* API */
    else
  #endif
  #ifdef CLUTTER_WINDOWING_WIN32
    if (clutter_check_windowing_backend (CLUTTER_WINDOWING_WIN32))
      /* use the clutter_win32_* API */
  #endif

This scheme allows us to ensure that the input system namespace is free
for us to use and select at run time in later versions of Clutter.