ImportC can now automatically run the preprocessor

[Walter Bright]

On 5/14/2022 7:12 PM, max haughton wrote:
> I will likely never compile a C dependency with ImportC directly, and I will 
> definitely never import a D module into a C project

Consider that one thing ImportC enables is C code calling D code. This is 
important if your project is a mix of C and D code.

I did not anticipate or think of this when ImportC was conceived. It was a 
Eureka moment to discover it just kinda fell out of the implementation (adding 
10 lines of code to close the circle!).