Allow controlling whether systemd is used or not

Currently this will switch the build system between using a plain D-BUS
service file, or a user controllable systemd unit that can be actively
stopped/started.

In most cases it is more desirable to use the systemd unit approach,
as plain D-BUS services cannot be controlled and are more difficult
to introspect via the log.

For fallback cases we'll have the plain D-BUS unit, and in future we
can use this to allow untying from systemd specifics.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

data/gamemoded.service.in

@@ -0,0 +1,10 @@
+[Unit]
+Description=gamemoded
+
+[Service]
+Type=dbus
+BusName=com.feralinteractive.GameMode
+ExecStart=@BINDIR@/gamemoded -l
+
+[Install]
+WantedBy=default.target

data/meson.build

@@ -2,15 +2,25 @@ data_conf = configuration_data()
 data_conf.set('BINDIR', path_bindir)
 data_conf.set('LIBEXECDIR', path_libexecdir)
 
-# Install the D-BUS service file
-configure_file(
-    input: 'com.feralinteractive.GameMode.service.in',
-    output: 'com.feralinteractive.GameMode.service',
-    configuration: data_conf,
-    install_dir: path_dbus_service_dir,
-)
+if with_systemd == true
+    # Install systemd user unit
+    configure_file(
+        input: 'gamemoded.service.in',
+        output: 'gamemoded.service',
+        configuration: data_conf,
+        install_dir: path_systemd_unit_dir,
+    )
+else
+    # Install the D-BUS service file
+    configure_file(
+        input: 'com.feralinteractive.GameMode.service.in',
+        output: 'com.feralinteractive.GameMode.service',
+        configuration: data_conf,
+        install_dir: path_dbus_service_dir,
+    )
+endif
 
-# Install the Polkit action file
+# Install the Polkit action file in all cases
 configure_file(
     input: 'com.feralinteractive.GameMode.policy.in',
     output: 'com.feralinteractive.GameMode.policy',