As discovered by Albin Söderqvist (see the commentary in his 'openttd'
patch [1]), the following command fails:

  guix download http://binaries.openttd.org/releases/1.6.0/openttd-1.6.0-source.tar.xz

with the following error:

--8<---------------cut here---------------start------------->8---
Starting download of /tmp/guix-file.sYMDJZ
From http://binaries.openttd.org/releases/1.6.0/openttd-1.6.0-source.tar.xz...
ERROR: download failed "http://binaries.openttd.org/releases/1.6.0/openttd-1.6.0-source.tar.xz" 307 "Temporary Redirect"
failed to download "/tmp/guix-file.sYMDJZ" from "http://binaries.openttd.org/releases/1.6.0/openttd-1.6.0-source.tar.xz"
guix download: error: http://binaries.openttd.org/releases/1.6.0/openttd-1.6.0-source.tar.xz: download failed
--8<---------------cut here---------------end--------------->8---

This happens because 'http-fetch' procedure from (guix build download)
module handles only 301 and 302 codes, while here we have 307 [2] [3].

The attached patch fixes this problem.

[1] http://lists.gnu.org/archive/html/guix-devel/2016-04/msg00445.html
[2] https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#3xx_Redirection
[3] https://tools.ietf.org/html/rfc2616#section-10.3.8