meson: allow building with elogind

Currently, the GDM meson build has a hard dependency on systemd.
However, GDM can function just fine if one is using elogind. This allows
a user to build GDM against libelogind and also disable the systemd
system and user units.
1 job for elogind-build in 5 minutes and 7 seconds (queued for 1 second)
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