scons: Adjust the warnings for VS.
Silence insignificant warnings so significant warnings have a chance to stand out. The only abundant warning that's not silenced here is "C4018: signed/unsigned mismatch", as it could hide security issues, so it's better to actually fix the code. Reviewed-by:Brian Paul <brianp@vmware.com> Reviewed-by:
Roland Scheidegger <sroland@vmware.com>