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