Use package-config output for -lsystemd correctly

In systemd 209, they deprecated -lsystemd-daemon in favor of
-lsystemd.  So we'd better actually look at the pkg-config output,
or we'll get warnings on newer distributions.

For some as-yet-unknown-to-me reason, setting CFLAGS so early makes
it so -O2 -g doesn't get added to it later.  So, adding it myself
later.  Perhaps a better fix here can be found.

Fixes 14072; bugfix on 0.2.6.2-alpha.  Based on a patch from h.venev