Easier building for Windows via ./do

Running './do windows' will display a warning about the current testing-only status of Windows builds, then build for Windows using the method documented @ https://github.com/cjdelisle/cjdns/commit/0d021678f740e74f1923d534de4540cfd79971a9

This will make building for Windows easier, as well as allow changes to the build command to be immediately reflected by buildbots and users who don't parse commits to changes to documentation. It will also allow updates on the status of windows builds to be displayed at build time.