The procedure ‘which’ from (guix build utils)
is used for two different purposes:

 1. for finding the absolute file name of a binary
    that we need to run during the build process

 2. for finding the absolute file name of a binary,
    for the target system (as in --target=TARGET),
    e.g. for substituting sh->/gnu/store/.../bin/sh,
    python->/gnu/store/.../bin/python

When compiling natively (SYSTEM=TARGET modulo nix/autotools differences),
this is perfectly fine.

However, when cross-compiling, we have a problem.
"which" looks in $PATH for binaries.  That's good for purpose (1),
but incorrect for (2), as the $PATH contains binaries from native-inputs
instead of inputs.

Admittedly, ‘which’ is simply very convenient (although incorrect
when cross-compiling), and we don't have the right tool ...
until now, that is (see patch)!

This patch adds an optional 'inputs' argument.  When it is present,
'which' will look in the bin and sbin subdirectories of the directories
in the 'inputs' alist.

Use it like this:

  (package [...]
    (arguments
     `(#:modules (guix build utils)
       #:phases
       (modify-phases %standard-phases
         (delete 'configure)
         (delete 'check)
         (add-after 'install 'wrap
           (lambda* (#:key outputs inputs #:allow-other-keys)
             (let ((growpart (string-append (assoc-ref outputs "out")
                                            "/bin/growpart")))
               (wrap-program growpart
                 `("PATH" ":" prefix (,(dirname (which "sfdisk" inputs))
                                      ,(dirname (which "readlink" inputs)))))))))))

(Examples comes from the "cloud-utils" package)
The only change is adding adding the 'inputs' argument.  Isn't that easy?

Alternative methods I've seen:
* (string-append (assoc-ref inputs "coreutils") "/bin/readlink")
* (let ((coreutils (assoc-ref inputs "coreutils")))
    (setenv "PATH" (string-append coreutils "/bin"))
    [code using (which "readlink")])
* (which "readlink") ; possibly incorrect when cross-compiling

I've tested this with "cloud-utils", though admittedly I didn't try to cross-compile
yet, and I've placed my adjusted "which" in a separate module to avoid having to rebuild
everything.  (The attached patch just modifies (guix build utils).)  I've written
a few tests, which pass.  I also documented the new functionality in the manual.

Currently incorrect uses of "which" can be fixed in a follow-up patch.

Thoughts?
Maxime.