Hi Marius,
Hi Tobias,

> > +         (add-before 'configure 'patch-installation-paths
> > +           (lambda* (#:key outputs #:allow-other-keys)
> > +             (let* ((out (assoc-ref outputs "out"))
> > +                    (doc (string-append out "/share/doc/" ,name "-" ,version)))
> > +               (substitute* "Makefile.in"
> > +                 (((string-append ".*\\$\\(DESTDIR\\)\\$\\("
> > +                                  "(config|pid|xfr|db)dir"
> > +                                  "\\).*"))
> > +                  "")  
> 
> Can you add a comment about what's going on here?

I agree... I'm scratching my head here. The ".*" is particularily worrying.
Doesn't that mean it will replace all the installation lines by nothing in the end?

Does that install anything still? :)

So now I've downloaded the source tarball and checked.

By extreme obscurity :), this only matches the "install -d" lines which are
creating empty directories (empty because of the configure-flags).

>  Would setting
> DESTDIR=(assoc-ref %outputs "out") in #:make-flags suffice?

DESTDIR is for installing into a tempdir.  After that is done the system
is supposed to "mv $tempdir /" and the software is usually written *not*
to expect DESTDIR to be still here when it runs.  PREFIX is something
else and probably what you had in mind.