Add `-mwin32` flag on Windows targets

See rust-lang/time#45.

I figured adding it to the defaults would be the most controllable way of implementing this fix. That way users can opt-out if it's inducing unwanted behavior. I'll reboot into Windows in a second to test this.

Edit: I can't exactly attest to the fix as Cargo silences the prints from `gcc-rs`, however it does build correctly even after removing the patch I made to `time`, so I presume it works.