Make inter-release --version output more useful.

Make inter-release --version output more useful.

Now, each unofficial build has a version "number" like 2.61a-19-58dd,
which indicates that it is built using the 19th change set
(in _some_ repository) following the "v2.61a" tag, and that 58dd
is a prefix of the commit SHA1.
* build-aux/git-version-gen: New file.
* configure.ac: Run it to set the version.
(AM_INIT_AUTOMAKE): Don't check NEWS here.
* 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.
* tests/Makefile.am ($(srcdir)/package.m4): Depend on Makefile,
not configure.ac, now that the version number changes automatically.

Ensure that $(VERSION) is up to date for dist-related targets.
* GNUmakefile: Arrange to rerun autoconf, if the version reported by
git-version-gen doesn't match $(VERSION), but only for dist targets.