ui/gtk: Get display refresh rate with GDK version 3.22 or later

ui/gtk: Get display refresh rate with GDK version 3.22 or later

Commit c4c00922cc introduced the use of the GdkMonitor API, which
was introduced in GTK+ 3.22:
https://developer.gnome.org/gdk3/stable/api-index-3-22.html#api-index-3.22

Unfortunately this break building with older versions, as on Ubuntu
Xenial which provides GTK+ 3.18:

  $ lsb_release -cd
  Description:    Ubuntu 16.04.5 LTS
  Codename:       xenial

  $ ./configure && make
  GTK support       yes (3.18.9)
  GTK GL support    no
  [...]
    CC      ui/gtk.o
  qemu/ui/gtk.c: In function ‘gd_vc_gfx_init’:
  qemu/ui/gtk.c:1973:5: error: unknown type name ‘GdkMonitor’
       GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
       ^
  qemu/ui/gtk.c:1973:27: error: implicit declaration of function ‘gdk_display_get_monitor_at_window’ [-Werror=implicit-function-declaration]
       GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
                             ^
  qemu/ui/gtk.c:1973:5: error: nested extern declaration of ‘gdk_display_get_monitor_at_window’ [-Werror=nested-externs]
       GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
       ^
  qemu/ui/gtk.c:1973:27: error: initialization makes pointer from integer without a cast [-Werror=int-conversion]
       GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
                             ^
  qemu/ui/gtk.c:2035:28: error: implicit declaration of function ‘gdk_monitor_get_refresh_rate’ [-Werror=implicit-function-declaration]
       refresh_rate_millihz = gdk_monitor_get_refresh_rate(monitor);
                              ^
  qemu/ui/gtk.c:2035:5: error: nested extern declaration of ‘gdk_monitor_get_refresh_rate’ [-Werror=nested-externs]
       refresh_rate_millihz = gdk_monitor_get_refresh_rate(monitor);
       ^
  cc1: all warnings being treated as errors
  qemu/rules.mak:69: recipe for target 'ui/gtk.o' failed
  make: *** [ui/gtk.o] Error 1

GTK+ provides convenient definition in <gdk/gdkversionmacros.h>
(already include by <gdk/gdk.h>) to check which API are available.

We only use the GdkMonitor API to get the monitor refresh rate.

Extract this code as a new gd_refresh_rate_millihz() function,
and check GDK_VERSION_3_22 is defined before calling its API.
If it is not defined, return 0. This is safe and fixes our build
failure (see https://travis-ci.org/qemu/qemu/builds/636992508).

Reported-by: Travis-CI
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200116115413.31650-1-philmd@redhat.com
Fixes: c4c00922cc (display/gtk: get proper refreshrate)
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>