在使用android-ndk-r4b用命令make APP=hello-jni编译hello-jni例子的时候，出现了以下错误信息：

Android NDK: APP variable defined to unknown applications: hello-jni
Android NDK: You might want to use one of the following:
build/core/main.mk:81: *** Android NDK: Aborting . Stop.

在查阅了OVERVIEW.TXT后，才发现r4b中编译的时候是以下步骤：

  1/ Place your native sources under $PROJECT/jni/...

  2/ Write $PROJECT/jni/Android.mk to describe your sources
     to the NDK build system

  3/ Optional: write $PROJECT/jni/Application.mk to describe your
     project in more details to the build system. You don't need
     one to get started though, but this allows you to target
     more than one CPU or override compiler/linker flags
     (see docs/APPLICATION-MK.TXT for all details).

  4/ Build your native code by running "$NDK/ndk-build" from your
     project directory, or any of its sub-directories.

照着做就编译成功了，如图：