look for an init system and auto install based on what is found

contrib/s6/meson.build

@@ -1,5 +1,3 @@
-s6 = get_option('s6')
-
 s6_config_data = configuration_data()
 s6_config_data.set('LIBEXECDIR', join_paths(get_option('prefix'), get_option('libexecdir'), meson.project_name()))
 
@@ -9,7 +7,9 @@ s6_config_data_file = configure_file(
     configuration: s6_config_data
 )
 
-if s6
+s6 = find_program ('s6-init', required: false)
+
+if s6.found()
     install_data(
         [s6_config_data_file, 'type', 'dependencies'],
         install_dir: join_paths(get_option('sysconfdir'), 's6', 'sv', 'tufmanager')