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
@@ -4229,7 +4229,7 @@ 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