I have just built src:dummy-systemd-dev for experimental.

The binary contains a single file[1] extracted from Debian's systemd binary
package. The file enables other package builds to query installation paths using
pkg-config(1).

Whilst I have not tested all of the Build-Depends: systemd sources in our
archive, manual installation of dummy-systemd-dev has enabled compilation of
alsa-utils and pipewire. Further testing is welcome. There may still be packages
for which this is insufficient.

Kudos and thanks to Tito for the idea.

Mark

[1] /usr/share/pkgconfig/systemd.pc