[PATCH v2] ui/gtk: Get display refresh rate with GDK version 3.22 or later

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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
Fixes: c4c00922cc (display/gtk: get proper refreshrate)
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v2: Improved commit description

Sorry I missed that, I only tested Nikola's patch on my workstation
which runs Fedora 30:

  $ pkg-config --modversion gtk+-3.0
  3.24.11

And Gerd tested on RHEL-7 which has a gtk version new enough for that.

This patch could get applied directly on master as a build fix to
fix the Travis-CI failures.
---
 ui/gtk.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/ui/gtk.c b/ui/gtk.c
index 7355d34fcf..d18892d1de 100644
--- a/ui/gtk.c
+++ b/ui/gtk.c
@@ -1961,6 +1961,23 @@ static GtkWidget *gd_create_menu_machine(GtkDisplayState *s)
     return machine_menu;
 }
 
+/*
+ * If available, return the refresh rate of the display in milli-Hertz,
+ * else return 0.
+ */
+static int gd_refresh_rate_millihz(GtkDisplayState *s)
+{
+#ifdef GDK_VERSION_3_22
+    GdkDisplay *dpy = gtk_widget_get_display(s->window);
+    GdkWindow *win = gtk_widget_get_window(s->window);
+    GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
+
+    return gdk_monitor_get_refresh_rate(monitor);
+#else
+    return 0;
+#endif
+}
+
 static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
                               QemuConsole *con, int idx,
                               GSList *group, GtkWidget *view_menu)
@@ -1968,10 +1985,6 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
     bool zoom_to_fit = false;
     int refresh_rate_millihz;
 
-    GdkDisplay *dpy = gtk_widget_get_display(s->window);
-    GdkWindow *win = gtk_widget_get_window(s->window);
-    GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
-
     vc->label = qemu_console_get_label(con);
     vc->s = s;
     vc->gfx.scale_x = 1.0;
@@ -2032,7 +2045,7 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
     vc->gfx.kbd = qkbd_state_init(con);
     vc->gfx.dcl.con = con;
 
-    refresh_rate_millihz = gdk_monitor_get_refresh_rate(monitor);
+    refresh_rate_millihz = gd_refresh_rate_millihz(s);
     if (refresh_rate_millihz) {
         vc->gfx.dcl.update_interval = MILLISEC_PER_SEC / refresh_rate_millihz;
     }
-- 
2.21.1