bug#69682: Opam should be using a minimal version of extlib

[Vivien Kraus]

Dear OCaml-aware Guix,

opam seems to imply that we have to use a minimal variant of ocaml-
extlib to build opam:

https://github.com/ocaml/opam/issues/5694

This is likely related to a build failure we are experiencing on the
gnome-team branch:

https://issues.guix.gnu.org/69677

http://ci.guix.gnu.org/build/3415343/log/raw

Best regards,

Vivien