build: deprecate fbdev backend

Rename the build option to "deprecated-backend-fbdev" so that a
previously configured build dir doesn't retain the old setting.
This is consistent with the existing "deprecated-wl-shell" option.
Make the option default to "false".

Print a warning when fbdev is force-enabled.

Signed-off-by: Simon Ser <contact@emersion.fr>
References: https://gitlab.freedesktop.org/wayland/weston/-/issues/581

libweston/backend-fbdev/fbdev.c

@@ -893,6 +893,8 @@ fbdev_backend_create(struct weston_compositor *compositor,
 		seat_id = param->seat_id;
 
 	weston_log("initializing fbdev backend\n");
+	weston_log("warning: the fbdev backend is deprecated, please migrate "
+		   "to the DRM backend\n");
 
 	backend = zalloc(sizeof *backend);
 	if (backend == NULL)

libweston/backend-fbdev/meson.build

@@ -1,7 +1,10 @@
-if not get_option('backend-fbdev')
+if not get_option('deprecated-backend-fbdev')
 	subdir_done()
 endif
 
+warning('Support for the deprecated fbdev backend is enabled.')
+warning('This feature will be removed in a future version.')
+
 config_h.set('BUILD_FBDEV_COMPOSITOR', '1')
 
 srcs_fbdev = [