Static checking by GCC 4.8.0.

Static checking by GCC 4.8.0.

* xfns.c (x_get_net_workarea, struct MonitorInfo, free_monitors)
(x_get_monitor_for_frame, x_make_monitor_attribute_list)
(x_get_monitor_attributes_fallback)
(x_get_monitor_attributes_xinerama)
(x_get_monitor_attributes_xrandr, x_get_monitor_attributes):
Define only if USE_GTK.
(free_monitors): Define only if HAVE_XINERAMA || HAVE_XRANDR.
(x_get_monitor_attributes_fallback): Omit unused locals.
(x_get_monitor_attributes_xinerama, Fx_display_monitor_attributes_list):
Use double, not float, to avoid mixed-mode floating point arithmetic.