Here I used this Android.mk file in jni/ folder.
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_PATH := $(call my-dir)
Have a look at the ndk documentation for prebuilts:
android-ndk/docs/PREBUILTS.html
You have to change
LOCAL_SHARED_LIBRARIES :=../lib/libpackext.so.1.0
to
LOCAL_SHARED_LIBRARIES := packext
Be sure that your folder containing the Android.mk for the packext module is named packext and can be found in in your NDK_MODULE_PATH.