Fix configure option --with-pkg-config-libdir is broken for cross compilation

--- a/configure	2021-10-17 17:12:23.000000000 +0200
+++ b/configure	2021-11-26 00:27:00.224815736 +0100
@@ -4255,7 +4255,7 @@ echo $ECHO_N "checking for first directo
 	cf_pkg_config_path=none
 	for cf_config in $cf_search_path
 	do
-		if test -d "$cf_config"
+		if test -n "$cf_config"
 		then
 			cf_pkg_config_path=$cf_config
 			break
