Configure: add back /WX to VC-WIN32.

We had /WX (treat warnings as errors) in VC-WIN32 for long time. At
some point it was somehow omitted. It's argued that it allows to
keep better focus on new code, which motivates the comeback...

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4718)