Enable systemd support by compiling against libsystemd-dev

Michael Prokoprequested to merge
mika/pkg-redis:debian/sid into debian/sid

By adding libsystemd-dev and pkg-config to Build-Depends, the build system automatically enables systemd support.

Only when systemd support is explicitly enabled, redis versions >=6.0.9-1 provide support for sd_notify() / NOTIFY_SOCKET / READY=1, otherwise the service is stuck in activating when used with Type=notify.

Closes: #977852

@lamby not sure whether you'd prefer to also explicitly use USE_SYSTEMD=yes in debian/rules. Maybe it's also worth considering switching to Type=notify in debian/bin/generate-systemd-service-files?

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