Meson runs a test to see if Sphinx works, and automatically disables it
on error. This can lead to the CI jobs skipping docs build without
maintainers noticing the problem. Use --enable-docs to force a fatal
error if Sphinx doesn't work on the jobs where we expect it to be OK.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20201102130926.161183-3-berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>