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 <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20200411182934[email protected]
Reviewed-by: Alex Bennée <[email protected]>