Building with `/W3` shows an awful lot of warnings on Windows, so it's
really hard to spot the more important ones. Since it is not possible
to override the hard-coded `/W3`, we drop it altogether, so MSVC uses
the default `/W1`. Users are encouraged to increase the warning level
via the environment variable `CFLAGS` before doing configure.
We also enable `/WX` (treat warnings as errors) for AppVeyor CI, using
`/W1` for now, since otherwise the build would fail.