Revert "Problem: debian has useless build dependency on systemd"
Revert "Problem: debian has useless build dependency on systemd"

This reverts commit 2cbfbb6d0879fdfdfbadb922a06e0b9bd482ba7a.

Re-add for systemd's pkg-config file, for unit install path.

Conflicts:
	zproject_debian.gsl

zeromq/zproject,paddor/zproject,taotetek/zproject,paddor/zproject,taotetek/zproject,zeromq/zproject,zeromq/zproject,taotetek/zproject,paddor/zproject