Warn about missing "mainSourceFile" only for executable configurations.
Warn about missing "mainSourceFile" only for executable configurations.

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