* Makefile.am (EXTRA_DIST, BUILT_SOURCES): Add $(top_srcdir)/.version.
($(top_srcdir)/.version, gen-tarball-version): New targets.
(dist-hook): Depend on 'gen-tarball-version'.
(.PHONY): Add 'gen-tarball-version'.
* build-aux/git-version-gen: New file, from Gnulib v0.1-1312-ga87d5e5c6.
* configure.ac: Use it in 'AC_INIT'. Use 'https' for the URL.