smlnj: Don't download on demand & build on Xcode.

Given superenv would allow `ENV.m32` (as suggested in issue #16350), this is my attempt to fix `smlnj`. I gave up [earlier](https://github.com/mxcl/homebrew/pull/12143#issuecomment-8022895).

But thanks to superenv, I was able to fix things and make it build on Xcode only, too.

**Don't pull unless superenv allows 32bit builds, too**