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.
`_WIN32` is a go, but the `#if` doesn't work like you think.  It seems to evaluate undefined macros to true, at least on my debian wheezy box (gcc 4.7.2).