configure: detect xen with --extra-cflags / --extra-ldflags Attached patch lets configure find xen headers and xen libs when called with --extra-cflags and --extra-ldflags options. Signed-off-by: Christoph Egger <Christoph.Egger@amd.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>