title: How to use FIND_JNI on cmake
body: <p>I'm trying to write a build for my project where I'm trying to replace autobuild, and I need to proper use FIND_JNI.</p>
  
  <p>I could make a simple build but it's not properly finding jni.h</p>
  
  <p>And I need to find a proper way (without a hack) to define the Java include as this needs to be portable to other users.</p>