title: Android NDK: Assertion failure: TARGET_PLATFORM is not defined
body: <p>When using NDK r5b, when I do a build in my jni directory using </p>
  
  <pre><code>$NDK_DIR/ndk-build 
  </code></pre>
  
  <p>it works fine.  But When I switch to r6b (just by setting $NDK_DIR differently) and run that same command,  I get </p>
  
  <pre><code>/usr/local/android-ndk-r6b/build/core/setup-toolchain.mk:20: *** Android NDK:  Assertion failure: TARGET_PLATFORM is not defined    .  Stop.
  </code></pre>
  
  <p>What do I need to do differently in r6b that I didn't need to do in r5b? </p>