configure: Pick sphinx-build-3 when available

The next commit will require a sphinx-build that uses Python 3.  On
some systems, sphinx-build is fine, on others you need to use
sphinx-build-3.  To keep things working out of the box on both kinds
of systems, try sphinx-build-3, then sphinx-build.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <87a75lqe8e.fsf@dusky.pond.sub.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>