title: how to include directory based on condition in cmake
body: <p>I am using windriver compile. I followed below steps</p>
  <pre><code>set(var_1)
  if(var_1)
   target_include_directories(cfar  PRIVATE ${APP_DIR}/z7b/variant_1)
  else
   target_include_directories(cfar  PRIVATE ${APP_DIR}/z7b/variant_2)
  endif()
  </code></pre>