Make inter-release --version output more useful.

Make inter-release --version output more useful.

Now, each snapshot has a version "number" like 6.9-219-g58ddd,
which indicates that it is built using the 219th change set
(in _some_ repository) following the "v6.9" tag, and that 58ddd
is a prefix of the commit SHA1.
* build-aux/git-version-gen: New file.
* configure.ac: Run it to set the version.
* Makefile.am (dist-hook): Arrange so that .version appears only
in distribution tarballs, never in a checked-out repository.
* .gitignore: Add .version here, too.  Just in case.

Signed-off-by: Jim Meyering <jim@meyering.net>