Add ImportC compiler to dmd

[Brian]

On Monday, 10 May 2021 at 01:13:25 UTC, Max Haughton wrote:
> ```
> #include <stdio.h>
> inline int square(int num) {
>     return num * num;
> }
>
> int main()
> {
>     printf("%d\n", square(3));
> }
> ```
> fails to link in C but not in C++.
>

Whether or not your code links in C depends on a number of 
factors. It is not as simple as you make it out to be.

Consider on my OpenBSD machine:
```
/home/brian/c $ gcc --version
gcc (GCC) 12.0.0 20210421 (experimental)
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  
There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A 
PARTICULAR PURPOSE.

/home/brian/c $ gcc -o square square.c
ld: error: undefined symbol: square
>>> referenced by square.c
>>>               /tmp//cc3wDuve.o:(main)
collect2: error: ld returned 1 exit status
/home/brian/c $ gcc -O2 -o square square.c
/home/brian/c $ ./square
9
```

~Brian