initial fixes for building kj/main.c++ utilities on mingw

Note, this isn't all that's needed to fix the mingw build (cross compiling from linux).  In particular, this only fixes what I needed to use kj/main.c++ (and dependencies).

I'm assuming that the desired way of dealing with utf-16 windows APIs is to keep everything internally as utf-8 (as advocated by http://utf8everywhere.org/).

With wider windows API usage, we'll eventually want to extract a common way of doing the utf-8 <-> utf-16 translation, rather than calling MultiByteToWideChar all over the place.  I'm not sure what that will look like.
Completely agree.  What do you think about having a "windows-sanity.h" (or whatever) where we organize all of these?

Then we can either (at your preference):
* Detect when windows.h has been included before capnp/kj headers and `#include "windows-sanity.h"` for users.  Common.h might be a good place to do this.  OR:
* Adopt a similar policy, where if you include windows.h and capnp, it's your business to also include "windows-sanity.h"

I can't decide which is the least harmful/confusing solution.