audit: Add systemd support

The main .service file is installed by audit.
We are using tmpfiles mechanisms to create the log directory, which
would not exists otherwise since /var/log points to /tmp

[Thomas: add explicit --disable-systemd when systemd is not used.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>