configure: Allow user to specify sphinx-build binary

configure: Allow user to specify sphinx-build binary

Currently we insist on using 'sphinx-build' from the $PATH;
allow the user to specify the binary to use. This will be
more useful as we become pickier about the capabilities
we require (eg needing a Python 3 sphinx-build).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200213175647.17628-2-peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>