Fixes <http://bugs.gnu.org/19278>.
Reported by Tomáš Čech <tcech@suse.cz>.
This allows 'guix --version' to return something that better represents
what version is being used.
* build-aux/build-self.scm (date-version-string): New procedure.
(build): Add #:version.
[builder]: Pass it to 'build-guix' as #:package-version.