build: use autobuild module to make build logs nicer

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.

bootstrap.conf

 # Bootstrap configuration.
 
-# Copyright (C) 2010-2012 Red Hat, Inc.
+# Copyright (C) 2010-2013 Red Hat, Inc.
 
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -20,6 +20,7 @@
 gnulib_modules='
 accept
 areadlink
+autobuild
 base64
 bind
 bitrotate

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 
-dnl Copyright (C) 2005-2012 Red Hat, Inc.
+dnl Copyright (C) 2005-2013 Red Hat, Inc.
 dnl See COPYING.LIB for the License of this software
 
 AC_INIT([libvirt], [1.0.1], [libvir-list@redhat.com], [], [http://libvirt.org])
@@ -122,6 +122,12 @@ AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_CPP
 
+dnl Setting AB_VERSION makes the 'autobuild' lines of configure output
+dnl slightly more useful
+if test -d $srcdir/.git && git --version >/dev/null 2>&1 ; then
+  AB_VERSION=`cd $srcdir && git describe --match 'v[[0-9]]*' 2>/dev/null`
+fi
+
 gl_EARLY
 gl_INIT