Modification to foreign import/export

[Simon Marlow]

> If we want to rule cpp as out of bounds (a very worthwhile 
> but difficult goal),
> then we need to think about how to avoid conditional compilation too.

I think the goal is not to avoid the use of cpp, but rather to avoid
mentioning it in the spec.  In practice, many people will be using cpp
(one way or another) when they write a C-library binding.  Conditional
compilation is a fact of life, I don't think it's worth trying to avoid
- but at least there's no need to mention it in the FFI spec.

Cheers,
	Simon