Speed up `dub`.

[ciechowoj via Digitalmars-d-learn]

On Monday, 7 March 2016 at 21:56:11 UTC, Seb wrote:
> Use ld.gold - it will speed up your linking quite dramatically!
>
> https://code.dawg.eu/reducing-vibed-turnaround-time-part-1-faster-linking.html

Thanks, it improves things a little. However I've just had idea 
that it should be possible to implement 'speculative dependency 
checking' for dub. What I mean by that is to start build process 
and dependency checking in parallel and then if something changes 
(with dependencies), apply the changes and restart the build 
process, and if dependencies are OK continue.