[Makefile] use double-colon rule for package_bin such that it can be overridden without warnings (close #5240)

Makefile

@@ -252,7 +252,7 @@ package_src:
 	curl -s https://raw.githubusercontent.com/Kentzo/git-archive-all/1.12/git-archive-all -o extra/git-archive-all
 	python extra/git-archive-all $(PACKAGE_OUT_DIR)/$(PACKAGE_FILE_NAME)_src$(PACKAGE_SRC_EXTENSION)
 
-package_bin:
+package_bin::
 	mkdir -p $(PACKAGE_OUT_DIR)
 	rm -rf $(PACKAGE_FILE_NAME) $(PACKAGE_FILE_NAME).tar.gz
 	# Copy the package contents to $(PACKAGE_FILE_NAME)

Makefile.win

@@ -36,7 +36,7 @@ CC_CMD=($(OCAMLOPT) $(ALL_CFLAGS) -c $< 2>tmp.cmi && $(FILTER)) || ($(FILTER) &&
 CC_PARSER_CMD=($(OCAMLOPT) -pp camlp4o $(ALL_CFLAGS) -c src/syntax/parser.ml 2>tmp.cmi && $(FILTER)) || ($(FILTER) && exit 1)
 endif
 
-package_bin:
+package_bin::
 	mkdir -p out
 	rm -rf $(PACKAGE_FILE_NAME) $(PACKAGE_FILE_NAME).zip temp.zip
 	# Copy the package contents to $(PACKAGE_FILE_NAME)