The systemd.pc file (for use by pkg-config) was moved from the systemd package to the systemd-dev package in version 256~rc2-1. Therefore, require either systemd-dev or a version of systemd before that version, in order to be guaranteed the presence of the systemd.pc file.
* debian/control (Source: mandos/Build-Depends): Add version qualifier "(< 256~rc2-1)" to "systemd".