A recent build failure made me realize that we could usefully add
a bit more information to configure output, for aid in analysis of
failed builds.  Pulling in the autobuild module merely adds these
four lines to configure output:

configure: autobuild project... libvirt
configure: autobuild revision... v1.0.1-113-g7a74eeaf
configure: autobuild hostname... myhost
configure: autobuild timestamp... 20130102T233543Z

which can be useful even if not using the Autobuild project to parse
build logs.

* bootstrap.conf (gnulib_modules): Add autobuild.
* configure.ac: Favor git version over release version, when available.