scons: Disable MSVC signed/unsigned mismatch warnings.

scons: Disable MSVC signed/unsigned mismatch warnings.

By default gcc ignores the issue, and as result code that mixes
signed/unsigned is so widespread through the code base that it ends up
being little more than noise, potentially obscuring more pertinent
warnings.

Maybe one day we enable the corresponding gcc warnings and cleanup, but
until then, this change disables them.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>