doc/sphinx: Make doxygen warn as error depend on meson werror flag

As seen previous times, with newer doxygen version we seem to be
generating warnings and with it to stop generating documentation
entirely as we have enabled warning as error in the doxygen
configuration file.

By default meson werror build option is not enabled, so users can still
generate documentation when building regularly, and when we'll update to
the same doxygen version we should be able to catch those errors up if
any of them will pile up in between.

Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>