Clean up the windowing system defines

Clean up the windowing system defines

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.