Output a warning if no main source file is defined.
Output a warning if no main source file is defined.

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