Fix: dub broken if compiled with absolute path DC
Fix: dub broken if compiled with absolute path DC

Code that was supposed to find working compiler always tried to
build absolute path from compiler name - even it was absolute
path already.

Abscissa/dub,alexeibs/dub,xentec/dub,D-Programming-Language/dub,rjframe/dub,Flamaros/dub,grogancolin/dub,p0nce/dub,Abscissa/dub-data-mod,schuetzm/dub,nazriel/dub,y12uc231/dub,dreamsxin/dub,gedaiu/dub,Geod24/dub,jeanbaptistelab/dub,jelmansouri/dub