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 when building using apple-gcc42 or llvm-gcc.
  - Boost.Context and Boost.Coroutine are only supported on x86_64.