Bug 5041: Missing Debug::Extra breaks build on hosts with systemd (#611)

Bug 5041: Missing Debug::Extra breaks build on hosts with systemd (#611)

* Bug 5041: Missing Debug::Extra breaks build on hosts with systemd

Master commit 6fa8c66 (i.e. Bug 5016 fix) relied on Debug::Extra added
by master commit (ccfbe8f) that was not ported to v4. The port of the
former master commit lacked the required piece of the latter commit.

The problem is invisible on hosts without a systemd package (that Squid
can find/use) and with Squids explicitly ./configured --without-systemd.

* "Minimum features" build test should be --without-systemd

* LDFLAGS were missing SYSTEMD_LIBS in builds with systemd support

Co-authored-by: Amos Jeffries <yadij@users.noreply.github.com>