[bug#57146] [PATCH] gnu: polkit: Fix cross-compiling

[Mathieu Othacehe <othacehe@gnu.org>]

Hello Andrew,

> The polkit package used to refer to docbook-xsl via inputs rather native inputs,
> which obviously doesn't work when cross-compiling.

Thanks for working on that. It looks like this patch is breaking host
compilation:

--8<---------------cut here---------------start------------->8---
error: in phase 'fix-manpage-generation': uncaught exception:
wrong-type-arg "string-append" "Wrong type (expecting ~A): ~S" ("string" #f) (#f) 
phase `fix-manpage-generation' failed after 0.0 seconds
Backtrace:
           9 (primitive-load "/gnu/store/k2p4qxc23g871rm5q50bcx45f3m…")
In guix/build/gnu-build-system.scm:
    906:2  8 (gnu-build #:source _ #:outputs _ #:inputs _ #:phases . #)
--8<---------------cut here---------------end--------------->8---

Also you are doing a lot of things at the same time: fixing indentation,
introducing gexps, fixing cross-compilation. It would be great if you
could do that in separate patches to ease the review process.

Thanks,

Mathieu