meson.build: allow overriding sysusers.d dir

[tmuehlbacher]

fixes builds for non-FHS systems like Nix.

[bluca]

This is read from pkgconf exactly for this use case, so that distribution can ship a custom pc file with their preferred locations, what's the reason why nix is not using that?

[tmuehlbacher]

This is read from pkgconf exactly for this use case, so that distribution can ship a custom pc file with their preferred locations, what's the reason why nix is not using that?

In Nix, the resulting files from a build are installed into one (or more) unique store paths that includes the hash of the package and all of its build inputs/parameters. So systemd cannot know ahead of time where polkit will have to install its outputs to.

Some examples paths:

/nix/store/jbc0ga700chv62qr7q5x28bxrpmxk04g-polkit-124/lib/sysusers.d/polkit.conf
/nix/store/n85xhkvxv8vhbshyqwwk5l5aiq4xldcc-systemd-minimal-255.2/share/polkit-1/actions/org.freedesktop.systemd1.policy

I've specifically implemented this change to be similar to the already existing systemdsystemunitdir meson option. There technically is still a programming error in the meson file because systemd_dep is defined inside of an if condition but then used unconditionally in this code path. With this fix you can at least work around this error by specifying both the system unit dir and the sysusers dir as meson options.

[bluca]

It has nothing to do with the systemd build though, pkgconfig is simply key/value pair and you can override it as needed. It doesn't make sense to me to reimplement all the hundreds of variables defined in the pc file, across all thousands of projects using it, all with custom config options. This is why we have pkgconfig in the first place. Just rewrite the pc file as needed.

[bluca]

This is how cross compilation already does it btw, and it works just fine for that case, so I am pretty sure you can do the same in nix too.

[tmuehlbacher]

Ok, using a different approach now, see NixOS/nixpkgs#295087
I am closing this PR as it seems that you already have a separate one to fix the build failure with #417