* guix-build.in, guix-download.in: Define `prefix' and `datarootdir', so that `guilemoduledir' expands to something meaningful.
* guix-download.in: New file. * configure.ac: Emit `guix-download' and make it executable. * Makefile.am (bin_SCRIPTS): Add `guix-download'. * po/POTFILES.in: Add `guix-download.in'.