Makefile, configure: Support building rST documentation

Add support to our configure and makefile machinery for building
our rST docs into HTML files.

Building the documentation now requires that sphinx-build is
available; this seems better than allowing half the docs to
be built if it is not present but having half of them missing.
(In particular it means that assuming that distros configured with
--enable-docs they'll get a helpful error from configure telling
them the new build dependency.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-10-peter.maydell@linaro.org
Message-id: 20190228145624.24885-10-peter.maydell@linaro.org