Add support for the new Wayland protocol xdg-system-bell

From the release notes:

    system bell - allowing e.g. terminal emulators to hand off system
    bell alerts to the compositor for among other things accessibility
    purposes

The new protocol is used when the new config option
bell.system=yes (and the compositor implements the protocol,
obviously).

The system bell is rung independent of whether the foot window has
keyboard focus or not (thus relying on compositor configuration to
determine whether anything should be done or not in response to the
bell).

The new option is enabled by default.

meson.build

@@ -179,6 +179,15 @@ else
   xdg_toplevel_icon = false
 endif
 
+if wayland_protocols.version().version_compare('>=1.38')
+  add_project_arguments('-DHAVE_XDG_SYSTEM_BELL', language: 'c')
+  wl_proto_xml += [wayland_protocols_datadir / 'staging/xdg-system-bell/xdg-system-bell-v1.xml']
+  xdg_system_bell = true
+else
+  xdg_system_bell = false
+endif
+
+
 foreach prot : wl_proto_xml
   wl_proto_headers += custom_target(
     prot.underscorify() + '-client-header',
@@ -414,6 +423,7 @@ summary(
     'IME': get_option('ime'),
     'Grapheme clustering': utf8proc.found(),
     'Wayland: xdg-toplevel-icon-v1': xdg_toplevel_icon,
+    'Wayland: xdg-system-bell-v1': xdg_system_bell,
     'utmp backend': utmp_backend,
     'utmp helper default path': utmp_default_helper_path,
     'Build terminfo': tic.found(),