meson: Add back libsystemd(-daemon) support

I accidentally removed it along with the toplevel config directory. It
might be useful for starting Xwayland via systemd.

Fixes: 23296633 "Drop config directory"
3 jobs for meson-libsystemd in 1 minute and 49 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #8113437
debian-buster

00:00:14

 
  Build And Test
passed #8113438
meson

00:01:34

passed #8113439
meson-noglamor

00:01:33