Avoid duplicate downloading, when rebuilding failed package like libgtk2

package/Makefile.autotools.in

@@ -132,7 +132,7 @@ TAR ?= tar
 $(BUILD_DIR)/%/.stamp_downloaded:
 # support make source-check/external-deps
 ifeq ($(MAKELEVEL),1)
-	$(WGET) -P $(DL_DIR) $($(PKG)_SITE)/$($(PKG)_SOURCE)
+	-$(Q)test -e $(DL_DIR)/$($(PKG)_SOURCE) || $(WGET) -P $(DL_DIR) $($(PKG)_SITE)/$($(PKG)_SOURCE)
 	$(if $($(PKG)_PATCH),$(WGET) -P $(DL_DIR) $($(PKG)_SITE)/$($(PKG)_PATCH))
 else
 	$(call MESSAGE,"Downloading")
@@ -395,3 +395,5 @@ endif
 endef
 
 # :mode=makefile:
+
+