title: Read environment variable in make file
body: <p>I have a environment variable set with name $MY_ENV_VARIABLE.</p>
  
  <p>How do I use this variable inside my makefile to (for example) include some source files?</p>
  
  <pre><code>LOCAL_SRC_FILES = $(MY_ENV_VARIABLE)/libDEMO.so
  </code></pre>
  
  <p>Something like above doesn't seem to work.</p>
  
  <p>Note: in my case this is needed for building with the Android NDK but I guess this applies to make in general.</p>