I've come up with this:

  Pacify unused function warning in xfns.c with GTK2

  * src/xfns.c (x_get_net_workarea, x_get_monitor_for_frame)
  (x_make_monitor_attribute_list, x_get_monitor_attributes_fallback):
  [HAVE_XINERAMA] (x_get_monitor_attributes_xinerama)
  [HAVE_XRANDR] (x_get_monitor_attributes_xrandr)
  (x_get_monitor_attributes): Fix #ifdefs around definitions to avoid
  unused function warnings regardless of GTK use (bug#46509).
  [HAVE_XRANDR] (x_get_monitor_attributes_xrandr): Undefine
  RANDR13_LIBRARY after it's been used.

But I'm not sure this pacifies Martin, so please suggest the best way of
writing this.

Thanks,

-- 
Basil