Add ImportC compiler to dmd

[12345swordy]

> On Sunday, 9 May 2021 at 22:41:23 UTC, Adam D. Ruppe wrote in 
> github:
>This appears completely useless to me. First, it doesn't "just 
>work" since you need to run cpp over the file before you can use 
>it, and since that expands macros, it must be done for each 
>different target, meaning it must be part of the build system. 
>This offers no advantage over the (not great) existing solutions 
>and shares many of their problems including creating huge files.

>Even if that just worked, C bindings tend to rely on the 
>preprocessor to do common and necessary tasks like defining 
>constants. Without that, you can't even reasonably use real 
>world C headers. Possibly a limited processor could translate 
>them, but even that data is gone after cpp goes through it, so 
>there's no real hope to recover it into a usable form.

>I think trying to run the cpp over a whole D file like dpp does 
>is problematic... but like something must be done. dstep's 
>approach actually does a reasonably good job, at the cost of 
>requiring a bit of hand editing and some setting up of other 
>files too.

>I do think there's some potential in integrating with dmd... but 
>this PR as it is looks like more harm than good. And I'm not 
>sure it is worth the time sink it will take to get it to a 
>positive point.
The advantage of having this built into the compiler itself is 
that you don't have to rely on a third party library for 
importing c functions into your d code. Accessibility is the key 
advantage here.

- Alex