Support for building tarballs

[Ludovic Courtès]

Some details about recent changes...

Commit fe12c34 added the ‘dist-package’ procedure: given a package
object, it returns another one that is basically the initial one plus an
“autoreconf” phase, and a ‘make distcheck’ phase in replacement of the
‘build’ phase.  Those phases are in the new (guix build gnu-dist) module.

IOW, ‘dist-package’ can be given a pristine checkout of a typical GNU
project, and it returns a package object that bootstraps it and builds
its source tarball.

The main goal is to allow Guix itself to have a ‘tarball’ job on Hydra
that runs ‘make distcheck’.

To that end, I packaged Guix in Guix–which means one can type
‘guix build guix’.  :-)

Then, the build-aux/hydra/guix.scm recipe roughly does

  (dist-package guix "/path/to/checkout")

The result can be seen at:

  http://hydra.gnu.org/jobset/guix/master

Running ./bootstrap currently fails, because Git is unavailable.  But as
soon as it’s available, that should work.

Feedback welcome!

Ludo’.