title: test if a variable is empty and give default value during qmake
body: <p>How can you test if a variable is empty or not defined in a <code>qmake</code> <code>.pro</code> file? I want to 
  be able to set up a default value if the variable is not defined.</p>
  
  <p>I tried</p>
  
  <pre><code>eval("VARIABLE" = ""){
       VARIABLE = test
  }
  
  eval("VARIABLE" = ""){
       message(variable is empty)
  }
  </code></pre>
  
  <p>but I still get the message "variable is empty".</p>