grey out the notifications opts when they weren't compiled in

In this case, we deactivate all of the notification checkboxes,
and set all relevant preferences and checkboxes to false. We also
put a tooltip on the header for this column, saying "This build of
wesnoth doesn't support desktop notifications, contact your
package manager."

data/gui/default/window/lobby_sounds_options.cfg

@@ -149,6 +149,7 @@
 								horizontal_alignment = "left"
 
 								[label]
+									id = "notification_label"
 									definition = "default"
 
 									label = _ "Desktop Notification:"

src/gui/dialogs/lobby_sounds_options.cpp

@@ -87,7 +87,16 @@ static void setup_item(const std::string & item, twindow & window)
 	setup_pref_toggle_button(item+"_sound", mp_ui_sounds::get_def_pref_sound(item), window);
 
 	// Set up the notification checkbox
-	setup_pref_toggle_button(item+"_notification", mp_ui_sounds::get_def_pref_notif(item), window);
+	ttoggle_button * notif = setup_pref_toggle_button(item+"_notification", mp_ui_sounds::get_def_pref_notif(item), window);
+
+	// Check if desktop notifications are available
+	if (!desktop::notifications::available()) {
+		notif->set_value(false);
+		notif->set_active(false);
+		preferences::set(item+"_notif", false);
+	} else {
+		notif->set_active(true);
+	}
 
 	// Set up the in_lobby checkbox
 	setup_pref_toggle_button(item+"_in_lobby", mp_ui_sounds::get_def_pref_lobby(item), window);
@@ -121,6 +130,11 @@ void tlobby_sounds_options::pre_show(CVideo& /*video*/, twindow& window)
 		setup_item(i, window);
 	}
 
+	if (!desktop::notifications::available()) {
+		tlabel * nlabel = &find_widget<tlabel>(&window, "notification_label", false);
+		nlabel->set_tooltip(_("This build of wesnoth does not include support for desktop notifications, contact your package manager"));
+	}
+
 	ttoggle_button * in_lobby;
 	in_lobby = &find_widget<ttoggle_button>(&window,"ready_for_start_in_lobby", false);
 	in_lobby->set_visible(twidget::tvisible::invisible);

src/mp_ui_sounds.cpp

@@ -168,7 +168,7 @@ bool get_def_pref_sound(const std::string & id) {
 }
 
 bool get_def_pref_notif(const std::string & id) {
-	return (id == "private_message" || id == "ready_for_start" || id == "game_has_begun");
+	return (desktop::notifications::available() && (id == "private_message" || id == "ready_for_start" || id == "game_has_begun"));
 }
 
 bool get_def_pref_lobby(const std::string & id) {