conf: template the set of with-foo module sets we install by default

Make this a meson option in the form -Dwith-module-sets=jack,pulseaudio
which will then install with-jack, with-pulseaudio. That's the default
setting.

And add a comment to the files themselves so it's more obvious what they
are there for, just an empty file is confusing.

media-session.d/meson.build

@@ -7,10 +7,12 @@ conf_files = [
   [ 'v4l2-monitor.conf', 'v4l2-monitor.conf' ],
   [ 'media-session.conf', 'media-session.conf' ],
   [ 'alsa-monitor.conf', 'alsa-monitor.conf' ],
-  [ 'with-jack', 'with-jack' ],
-  [ 'with-pulseaudio', 'with-pulseaudio' ],
 ]
 
+foreach opt : get_option('with-module-sets')
+  conf_files += [ [ 'with-module-set.in', 'with-@0@'.format(opt) ] ]
+endforeach
+
 foreach c : conf_files
   configure_file(input : c.get(0),
                  output : c.get(1),

media-session.d/with-module-set.in

@@ -0,0 +1,5 @@
+# This file is part of Media Session @VERSION@
+#
+# Add this file to @MEDIA_SESSION_CONFIG_DIR@/media-session.d/
+# to load the corresponding module list in the in the media-session.conf
+# session.modules[<filename>] array.

meson_options.txt

@@ -29,3 +29,8 @@ option('systemd-user-service',
 option('systemd-user-unit-dir',
        type : 'string',
        description : 'Directory for user systemd units (defaults to /usr/lib/systemd/user)')
+option('with-module-sets',
+	description : 'Extra modules sets to enable on install (see media-session.conf)',
+	type : 'array',
+	choices : ['alsa', 'jack', 'pulseaudio'],
+	value : ['jack', 'pulseaudio'])