While we don't support prefix being != /usr, and this is hardcoded
all over the place, variables in pkg-config file are expected
to have overridable base directory.
This is important for at least the following two use cases:
- Installing projects to non-FHS package-specific prefixes for Nix-style
package managers. Of course, it is then their responsibility
to ensure systemd can find the service files.
- Installing to local path for development purposes.
This is a compromise between running a program from a build directory,
and running it fully installed to system prefix.
You will not want to write to system prefix in either case.
For more information, see also
https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/
Fixes https://github.com/systemd/systemd/issues/18082
Partially reverts 6e65df89c3
Jan Tojnar
60bce7c6d9