docs: how to make find_program() search additional directories
docs: how to make find_program() search additional directories

Fixes #1702

jeandet/meson,aaronp24/meson,mesonbuild/meson,jeandet/meson,MathieuDuponchelle/meson,becm/meson,jpakkane/meson,aaronp24/meson,jeandet/meson,mesonbuild/meson,mesonbuild/meson,ernestask/meson,wberrier/meson,fmuellner/meson,wberrier/meson,pexip/meson,fmuellner/meson,jeandet/meson,becm/meson,thiblahute/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,fmuellner/meson,becm/meson,fmuellner/meson,QuLogic/meson,jpakkane/meson,MathieuDuponchelle/meson,rhd/meson,rhd/meson,trhd/meson,wberrier/meson,mesonbuild/meson,thiblahute/meson,wberrier/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,wberrier/meson,pexip/meson,becm/meson,wberrier/meson,thiblahute/meson,thiblahute/meson,rhd/meson,rhd/meson,mesonbuild/meson,QuLogic/meson,ernestask/meson,MathieuDuponchelle/meson,thiblahute/meson,pexip/meson,ernestask/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,becm/meson,pexip/meson,aaronp24/meson,jeandet/meson,fmuellner/meson,aaronp24/meson,ernestask/meson,pexip/meson,mesonbuild/meson,trhd/meson,rhd/meson,ernestask/meson,rhd/meson,ernestask/meson,thiblahute/meson,becm/meson,MathieuDuponchelle/meson,aaronp24/meson,aaronp24/meson,ernestask/meson,trhd/meson,becm/meson,becm/meson,trhd/meson,jeandet/meson,ernestask/meson,QuLogic/meson,pexip/meson,pexip/meson,trhd/meson,fmuellner/meson,aaronp24/meson,wberrier/meson,trhd/meson,jeandet/meson,thiblahute/meson,QuLogic/meson,fmuellner/meson,jpakkane/meson,trhd/meson,pexip/meson,jpakkane/meson,ernestask/meson,fmuellner/meson,jpakkane/meson,aaronp24/meson,MathieuDuponchelle/meson,trhd/meson,QuLogic/meson,MathieuDuponchelle/meson,QuLogic/meson,jeandet/meson,thiblahute/meson,jeandet/meson,becm/meson,jpakkane/meson,becm/meson,jpakkane/meson,thiblahute/meson,mesonbuild/meson,QuLogic/meson,rhd/meson,fmuellner/meson,MathieuDuponchelle/meson,aaronp24/meson,trhd/meson,rhd/meson,mesonbuild/meson,wberrier/meson