configure: Honour --disable-werror for Sphinx

configure: Honour --disable-werror for Sphinx

If we are not making warnings fatal for compilation, make them
non-fatal when building the Sphinx documentation also.  (For instance
Sphinx 3.0 warns about some constructs that older versions were happy
with, which is a build failure if we use the warnings-as-errors
flag.)

This provides a workaround at least for LP:1872113.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200411182934.28678-2-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>