Build gnome-keyring against libsystemd

With this change, systemd user units gnome-keyring-daemon.service and gnome-keyring-daemon.socket will be installed, and the daemon itself will support systemd socket activation.

The knock-on effect is that pam_systemd will no longer spawn the daemon directly when it is not running – instead, it will rely on systemd to do that. This allows gnome-initial-setup-copy-worker.service to be explicitly sequenced Before=gnome-keyring-daemon.service, avoiding a race between the copy-worker copying the new user's keyring from /run/gnome-initial-setup, and gnome-keyring-daemon starting up.

Pass --with-systemd explicitly to avoid silently falling back to building without systemd support, as in the past.

This is the configuration used by every other distribution I checked: Fedora, Debian, Arch, Ubuntu.

gnome-initial-setup!249 (merged) gnome-initial-setup!250 (merged) gnome-keyring#137