title: cmake add_definitions and COMPILE_DEFINITIONS, how to see them
body: <p>I want to see what the current set of compiler definitions is in my CMake file.  Ones automatically specified and the ones I'd added would be great. The <code>COMPILE_DEFINITIONS</code> macro doesn't appear to contain -- despite what the documentation says.</p>
  
  <p>For example, in the below setup the message never includes <code>GUI_BUILD</code></p>
  
  <pre><code>add_definitions( -DGUI_BUILD )
  message( "COMPILE_DEFINITIONS = ${COMPILE_DEFINITIONS}" )
  </code></pre>
  
  <p>I don't need to see them in their final form, I just want a quick output to help verify that everything has been configured correctly.</p>