Automatically download ICU, eventually make `--with-intl` on by 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

Just thinking out loud here: is this the right place to do the download? should it at least be in a separate utility file called by `configure` or even not called automatically at all and have `configure` complain at you if you haven't downloaded it? Seems slightly code-smelly to have a download taking place in here, but perhaps there are precedents already set inside `configure`?