> Modulo typos, and indentation being weird because 'phone,
> 
> (arguments
>  (list
>   #:make-flags
>   #~(list (string-append "HELLO=" #$output "/bin/hello"))
>   #:phases
>   #~(modify-phases %standard-phases
>      (add-after 'unpack 'option-one
>       (lambda _
>        (substitute* "foo.c"
>         (("hello")
>          (string-append #$output "/bin/hello")))))
>      (add-after 'unpack 'option-two
>       (lambda* (#:key outputs #:allow-other-keys
>        (let ((out (assoc-ref outputs "out")))
>         (substitute* "foo.c"
>          (("hello")
>          (string-append out "/bin/hello"))))))))))

Thanks all. I got that part working. Unfortunately I found out that
firefox/icecat tries to find native messaging manifests only from a
hardcoded /usr/lib/... path so packaging a native messaging host
manifest is impossible without patching the browser (as far as I know).

- Saku Laesvuori