Support elogind for sd_notify

[Julien-Elie]

m4/systemd.m4 comes from https://github.com/rra/rra-c-util where this PR belongs (@rra). libelogind could indeed be looked at, besides libsystemd and libsystemd-daemon.

The comments will need being updated to reflect the change in the final commit. It may also be the right time to remove the _DAEMON string from the name of the macro, as suggested in its comment.

[rra]

I don't understand what problem you're trying to solve with this change. sd_notify exists only to notify a systemd init system that a daemon has started. libelogind is a compatibility library for building software on systems that do not use systemd. If the system doesn't use systemd, there's nothing to notify, and thus the default fallback behavior when libsystemd is not found of stubbing out sd_notify is correct, I think.

Under what situations is the call to libelogind helpful?