pkg-config: make prefix overridable again

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 6e65df89c348242dbd10036abc7dd5e8181cf733

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