Ludovic, as a follow-up to the short chat on #guix:
if the problem can not be patched in the guile module of gnutls, it appears that http://git.savannah.gnu.org/cgit/guile.git/tree/module/system/base/target.scm could be a source of the problem? At the moment I'm trying to patch (or rather sed/awk) in gnutls possible occurences of the tripplet appearing with the GNU_... in pkgsrc. Thanks, ng0