configure: add --version flag

Standard autoconf scripts include a --version flag so people can easily
query things.  Add this to qemu's configure so it too can integrate with
build systems that have standard autotool helpers.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

configure

@@ -494,6 +494,8 @@ for opt do
   case "$opt" in
   --help|-h) show_help=yes
   ;;
+  --version|-V) exec cat $source_path/VERSION
+  ;;
   --prefix=*) prefix="$optarg"
   ;;
   --interp-prefix=*) interp_prefix="$optarg"