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.
I think the preferred form is `_WIN32` (one underscore, not two). I believe both MinGW and MSVC will define `_WIN32` but only MinGW will do `__WIN32`, so we might as well use the former.

Nitpick: I prefer `#if` rather than `#ifdef` for these. (Compilers always define these macros to 1, and undefined macros are already treated as having the value 0.)