Bug 1762484 - Enable most of the same warnings on Windows as on other platforms. r=firefox-build-system-reviewers,andi

This moves the manual addition of some flags from old-configure to
python configure, and disables the set of flags that still trigger a
bunch of warnings (which -Werror turns into bustage).

Differential Revision: https://phabricator.services.mozilla.com/D144406