Add ImportC compiler to dmd

[matheus]

On Friday, 14 May 2021 at 05:27:27 UTC, Walter Bright wrote:
> As for the user having options, that's why we have multiple D 
> compilers. ImportC isn't part of the D language, and so 
> differing flavors of C support is within reason.

I hope this not lead to a fragmentation. Current we already have 
this case where if you want a faster compiling use DMD, but for 
production/optimization/reliability  go with GDC or LDC.

Matheus.