* guix.scm: Fill out home-page, synopsis, description, and license fields.
* guix.scm: Use Guile-Git to find the list of source files, and use those files for the source field of the package.
* build-aux/git-version-gen: New file. * .gitignore: Do not ignore it. * configure.ac: Use it to generate the version string. * guix.scm: Ditto. * Makefile.am: Make .tarball-version.