systemd: base "ListenStream" on build options

Make `mpd.socket` configurable based on build options, so that e.g.
`ListenStream=6600` is not added if `tcp` option is not enabled.

systemd/meson.build

@@ -1,5 +1,15 @@
 systemd_unit_conf = configuration_data()
 systemd_unit_conf.set('prefix', get_option('prefix'))
 
+systemd_socket_conf = configuration_data()
+listen_streams = []
+if get_option('local_socket')
+  listen_streams += 'ListenStream=%t/mpd/socket'
+endif
+if get_option('tcp')
+  listen_streams += 'ListenStream=6600'
+endif
+systemd_socket_conf.set('listen_streams', '\n'.join(listen_streams))
+
 subdir('system')
 subdir('user')

systemd/system/meson.build

@@ -12,8 +12,10 @@ if systemd_system_unit_dir == ''
   systemd_system_unit_dir = join_paths(get_option('prefix'), 'lib', 'systemd', 'system')
 endif
 
-install_data(
-  'mpd.socket',
+configure_file(
+  input: 'mpd.socket.in',
+  output: 'mpd.socket',
+  configuration: systemd_socket_conf,
   install_dir: systemd_system_unit_dir,
 )
 

systemd/system/mpd.socket.in

@@ -1,6 +1,5 @@
 [Socket]
-ListenStream=%t/mpd/socket
-ListenStream=6600
+@listen_streams@
 Backlog=5
 KeepAlive=true
 PassCredentials=true

systemd/user/meson.build

@@ -12,9 +12,10 @@ if systemd_user_unit_dir == ''
   systemd_user_unit_dir = join_paths(get_option('prefix'), 'lib', 'systemd', 'user')
 endif
 
-# copy the system socket unit to the "user" directory
-install_data(
-  join_paths('..', 'system', 'mpd.socket'),
+configure_file(
+  input: join_paths('..', 'system', 'mpd.socket.in'),
+  output: 'mpd.socket',
+  configuration: systemd_socket_conf,
   install_dir: systemd_user_unit_dir,
 )