Improve diagnostic messages for pkg-config.
Improve diagnostic messages for pkg-config.

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