configure: Allow systemd service installation without systemd.pc

configure: Allow systemd service installation without systemd.pc

If the systemdsystemunitdir got passed anyway as a variable, we don't
need the systemd.pc dependency anymore and allow building without a
systemd build-dependency.

Relevant in Alpine Linux, where we allowed systemd services to be
(sub-)packaged (e.g. for downstreams like postmarketOS), but don't have
systemd pacakged in Alpine itself, yet.

Closes: https://github.com/alsa-project/alsa-utils/pull/309
Signed-off-by: Achill Gilgenast <achill@achill.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>