> I think we’ll have to cheat by passing --build=i686-pc-gnu and removing
> --host.  This is actually what I did in Nixpkgs:
>  https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/gnu/hurd/default.nix

I will try that right away

>  Then the ‘build’ phase must be skipped, and the ‘install’ phase must
>  just do ‘make install-headers’.

That's already what's happening in the hurd-headers recipe in hurd.scm which xhurd-headers inherits from :-)

Manolis