Obviously all this would need to be documented before actually
apply such a patch. 

I've tested this patch by building e.g. gmsh (which I did not already
have in the store)::

  $ guix build --source=transitive gmsh
  [...]
  <disconnect network>
  $ guix build gmsh

The build succeeded after building the glu, glproto, fltk, and
xf86vidmodeproto inputs from source.  I haven't yet tried making a
change that would require rebootstrapping while offline.  I think the
idea is that this should work.

What I'd like to get some feedback on:

1. Adding an optional argument to `guix build --source` currently breaks
   `guix build -S foo`, since the option parsing thinks "foo" is the
   argument for -S.  I'm not sure how this could be reconciled.

2. There might be clearer user/developer-facing names for the arguments.

3. Am I understanding the use-case?

Thanks,

-- 
Eric Bavier