ImportC can now automatically run the preprocessor

[Walter Bright]

Interesting. I note that this is inserting C expressions and statements into D 
functions. ImportC works at the declaration level - i.e. inserting C symbols. I 
expect there may be impedance mismatch problems by putting it at a lower level. 
In fact I'm certain that a redo of how the C semantics are handled in the 
compiler would be necessary.