Intl: Automatically download ICU if requested. Reduce size. Keep `--with-intl=none` the default. 

Background: see joyent#7676 of course

* [x] decide which variant is on by default
* [x] make it on by default
* [x] test more failure cases of downloading
* [x] add md5 sum
* [x] test more platforms

It seemed better to keep the logic all in one place (`configure`). I was trying not to make `vcbuild.bat` make the policy decision, but just call through to `configure`. Is it better to put the default in two places?