[Dovecot] add basic systemd support to dovecot

[Timo Sirainen]

On Mon, 2010-10-25 at 17:47 +0100, Timo Sirainen wrote:
> > +if HAVE_SYSTEMD
> > +dovecot_SOURCES += sd-daemon.c
> > +noinst_HEADERS += sd-daemon.h
> > +endif
> 
> I don't think this is portable.

Or looks like it's already used in lib-sql/Makefile.am. Maybe it works
fine then. Or probably doesn't matter anyway since this is Linux-only
code and GNU make is always used there. So just keep this as is. :)