title: Qt pro file conditional libs
body: <p>I am trying to setup a pro file where I want it to only include a specific library if a variable is defined. I already have this variable passed into qmake via the command line by appending DEFINES+=VARIABLE_NAME.</p>
  
  <p>I can use VARIABLE_NAME to conditionally compile parts of my code but can I have a condition to detect VARIABLE_NAME in the pro file and only include a library if it exists?</p>
  
  <p>Thanks,
  Alan</p>