Since commit 5f71eac0 the Sphinx tool is required
to build the rST documentation.

This fixes:

 $ ./configure --enable-docs

 ERROR: User requested feature docs
        configure was not able to find it.
        Install texinfo, Perl/perl-podlators and python-sphinx

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190715174817.18981-3-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>