Add ImportC compiler to dmd

[Walter Bright]

On 5/10/2021 3:57 AM, Dibyendu Majumdar wrote:
> This seems like a good step forward. I would recommend adding an in-built C  
> pre-processor. Maybe port something small like this one:
> 
> https://github.com/rui314/chibicc/blob/main/preprocess.c
> 
> Many people write single header file C programs. With this enhancement it will 
> be easy to use all of that code in D.

Thanks for the tip! Unfortunately, the file is not a C preprocessor in all it's 
hideous detail, so it'll be a problem. Fortunately, we do have a C preprocessor:

https://github.com/DigitalMars/dmpp