configure: use command -v rather than which(1).

configure

@@ -106,7 +106,7 @@ done
 
 _which()
 {
-	x="$(which "$1" 2>/dev/null)"
+	x="$(command -v "$1" 2>/dev/null)"
 	if [ -n "$x" ]; then
 		echo "$x"
 		return 0