configure.ac: add explicit enable/disable for systemd-login support

Otherwise, auto-enable depending on whether the system has the necessary
libraries.

[Updated help text as per pq suggestion -- bwh]

Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 file changed