Porting D to custom OS

[IGotD-]

On Saturday, 22 February 2020 at 15:18:09 UTC, kinke wrote:
>
> I'd suggest to first hack the compiler, so that it doesn't 
> predefine the host OS, but a new version for your OS (and check 
> whether predefining `version (Posix)` is appropriate or not). 
> That way, you'll hit static asserts when compiling 
> druntime/Phobos, where OS-specific parts don't handle the new 
> OS yet. Once all compile errors are fixed, you can then move on 
> to compiling and linking the druntime/Phobos test runners, 
> fixing all linker errors along the way, and finally fixing all 
> failing unittests, possibly requiring compiler codegen 
> adaptations.

That's sound like a start. Is there some kind of automation 
behind this based on the target triple (-mtriple) or just some 
kind of hardcoded default versions identifiers somewhere in the 
source code?