configure: disable Archipelago by default and warn about libxseg GPLv3 license

libxseg has changed license to GPLv3.  QEMU includes GPL "v2 only" code
which is not compatible with GPLv3.  This means the resulting binaries
may not be redistributable!

Disable Archipelago (libxseg) by default to prevent accidental license
violations.  Also warn if linking against libxseg is enabled to remind
the user.

Note that this commit does not constitute any advice about software
licensing.  If you have doubts you should consult a lawyer.

Cc: Chrysostomos Nanakos <cnanakos@grnet.gr>
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Message-id: 1428587538-8765-1-git-send-email-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
