handle empty extensions list

extensions/Makefile.in

@@ -22,18 +22,9 @@ VPATH		= @srcdir@
 
 include $(DEPTH)/config/autoconf.mk
 
+ifdef MOZ_EXTENSIONS
 DIRS=$(MOZ_EXTENSIONS)
-#ifdef ClientWallet
-#DIRS += wallet
-#endif
-#
-#ifdef ENABLE_PICS
-#DIRS += pics
-#endif
-#
-#ifdef NECKO
-#DIRS += cookie
-#endif
+endif
 
 include $(topsrcdir)/config/config.mk