Another fix for compiles on an empty 'po' directory.

src/nls-global.mk

-# $PostgreSQL: pgsql/src/nls-global.mk,v 1.13 2008/05/17 20:24:05 petere Exp $
+# $PostgreSQL: pgsql/src/nls-global.mk,v 1.14 2008/05/17 21:27:12 momjian Exp $
 
 # Common rules for Native Language Support (NLS)
 #
@@ -78,7 +78,7 @@ uninstall-po:
 
 clean-po:
 	$(if $(MO_FILES),rm -f $(MO_FILES))
-	@rm -f $(addsuffix .old, $(PO_FILES))
+	@$(if $(PO_FILES),rm -f $(addsuffix .old, $(PO_FILES)))
 	rm -f po/$(CATALOG_NAME).pot