Hi,

as mentioned in IRC I have begun porting Guix to NetBSD (with the path
taken not yet decided upon, just plain building guix itself for now).

Glibc provides argp. Arguably we don't have to check for argp because
Guix targets glibc. But I am quiete certain that there will be people
who will attempt to do what I am doing and run into this.

I haven't tested this, but semantically it should check out. I don't
know how much changed in guix's bootstrap but configure.ac should still be
the right place for this patch.