How to solve compile warning: skipping <> — package not selected when execute OPKG compilation

I am newbie for openWrt and OPKG.

I am trying to compile a helloworld example, but when I execute the command:

make package/helloworld/compile V=99

I got below error:

WARNING: skipping libgfortran -- package not selected
WARNING: skipping ldd -- package not selected
WARNING: skipping ldconfig -- package not selected
make[2]: Leaving directory '/home/ubuntu/customer-test/lmu5541_sdk/openwrt/package/libs/toolchain'
make[2]: Entering directory '/home/ubuntu/customer-test/lmu5541_sdk/openwrt/package/helloworld'
WARNING: skipping helloworld -- package not selected
make[2]: Leaving directory '/home/ubuntu/customer-test/lmu5541_sdk/openwrt/package/helloworld'
make[1]: Leaving directory '/home/ubuntu/customer-test/lmu5541_sdk'

There are only above Makefile and src folder in package/helloworld. The Makefile in package/helloword is:

include $(TOPDIR)/rules.mk

PKG_NAME:=helloworld
PKG_VERSION:=1.0.0
PKG_RELEASE:=1

PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)-$(PKG_VERSION)

include $(INCLUDE_DIR)/package.mk

define Package/helloworld
	SECTION:=utils
	CATEGORY:=Utilities
	TITLE:=Helloworld Application
endef

CONFIGURE_VARS+= \
        CC="$(TOOLCHAIN_DIR)/bin/$(TARGET_CC)"

define Package/helloworld/description
	Simple hello world package
endef

define Build/Prepare
	mkdir -p $(PKG_BUILD_DIR)
	$(CP) ./src/* $(PKG_BUILD_DIR)/
endef


#define Build/Compile
#	$(call Build/Compile/Default)
#	$(TARGET_CC) $(HELLO_FLAGS) -o $(PKG_BUILD_DIR)/helloworld src/helloworld.c
#endef

define Package/helloworld/install
	$(INSTALL_DIR) $(1)/bin
	$(INSTALL_BIN) $(PKG_BUILD_DIR)/helloworld $(1)/bin/
endef

$(eval $(call BuildPackage,helloworld))

Could you please tell me how to solve this problem?