Boost: simplify universal handling and other improvements.

* Use Homebrew standard universal handling.
* Handle whether to build MPI at bootstrap.
  - Also improve parameter construction.
  - Use CPU.is_32_bit? instead of accessing CPU.bits directly.
* Add more caveats:
  - Boost.Log is disabled on Snow Leopard.
  - Boost.Context and Boost.Coroutine are not built in universal build.

WIP.