build: Allow disabling the X11 session

This is consistent with gnome-session and the rest of the GNOME
session stack.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/396>

data/meson.build

@@ -2,14 +2,19 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+have_x11 = get_option('x11')
+
 session_desktop_base = 'gnome-classic'
 
 session_desktops = [
   session_desktop_base,
-  session_desktop_base + '-xorg',
   session_desktop_base + '-wayland',
 ]
 
+if have_x11
+  session_desktops += [session_desktop_base + '-xorg']
+endif
+
 foreach name : session_desktops
   session_desktop = name + '.desktop'
   if name.endswith('-xorg')

meson.options

@@ -20,3 +20,9 @@ option('classic_mode',
   value: false,
   description: 'Enable installing data files for classic mode.'
 )
+
+option('x11',
+  type: 'boolean',
+  value: true,
+  description: 'Enable X11 session support.'
+)