ui: deprecate use of GTK 2.x in favour of 3.x series

The GTK 3.0 release was made in Feb, 2011:

  https://blog.gtk.org/2011/02/10/gtk-3-0-released/

That will soon be 7 years ago, which is enough time to consider
the 3.x series widely supported.

Thus we deprecate the GTK 2.x support, which will allow us to
delete it in the last release of 2018. By this time, GTK 3.x
will be almost 8 years old.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 20171212113440.16483-1-berrange@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

configure

@@ -5629,6 +5629,12 @@ if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
 fi
 
+if test "$gtkabi" = "2.0"; then
+    echo
+    echo "WARNING: Use of GTK 2.0 is deprecated and will be removed in"
+    echo "WARNING: future releases. Please switch to using GTK 3.0"
+fi
+
 if test "$supported_cpu" = "no"; then
     echo
     echo "WARNING: SUPPORT FOR THIS HOST CPU WILL GO AWAY IN FUTURE RELEASES!"

qemu-doc.texi

@@ -2587,6 +2587,15 @@ they were first deprecated in the 2.10.0 release.
 What follows is a list of all features currently marked as
 deprecated.
 
+@section Build options
+
+@subsection GTK 2.x
+
+Previously QEMU has supported building against both GTK 2.x
+and 3.x series APIs. Support for the GTK 2.x builds will be
+discontinued, so maintainers should switch to using GTK 3.x,
+which is the default.
+
 @section System emulator command line arguments
 
 @subsection -tdf (since 1.3.0)

ui/gtk.c

@@ -2248,6 +2248,11 @@ void gtk_display_init(DisplayState *ds, bool full_screen, bool grab_on_hover)
         exit(1);
     }
 
+#if !GTK_CHECK_VERSION(3, 0, 0)
+    g_printerr("Running QEMU with GTK 2.x is deprecated, and will be removed\n"
+               "in a future release. Please switch to GTK 3.x instead\n");
+#endif
+
     s->window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
 #if GTK_CHECK_VERSION(3, 2, 0)
     s->vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);