build-sys: Add support for newer systemd without compatibility libs

build-sys: Add support for newer systemd without compatibility libs

In newer versions of systemd some libraries were combined for the sake of
general simplicity.

This change checks against the newer name first and avoids separate pkgconfig
checks if it's found. We probably want to keep support for the older library
names for some time. systemd does allow for the shipping of compatibility
pkgconfig files to not break downstream code like ourselves which is why this
likely hasn't been "fixed" until now.

With this change we no longer rely on systemd having been built with those
compatibility pkgconfig files.