title: file(GENERATE OUTPUT ...) usage examle
body: <p>Good morning!</p>
  
  <p>Do someone have example of usage the "file(GENERATE OUTPUT" cmake closure:</p>
  
  <p>file(GENERATE OUTPUT output_file
       
       [CONDITION expression])</p>
  
  <p>Could I use one to generate source code files by template?</p>
  
  <p>I was unable to font out an answer in the official documentation:
  <a href="https://cmake.org/cmake/help/v3.0/command/file.html" rel="nofollow noreferrer">https://cmake.org/cmake/help/v3.0/command/file.html</a></p>
  
  <p>and in the Internet</p>
  
  <p>I have source files like</p>
  
  <h2>config.template :</h2>
  
  <p>config: 
     param1: ${PARAM_1_VALUE}</p>
  
  <hr>
  
  <p>and I wish to replace ${PARAM_1_VALUE} with some cmake variable value and get the corresponding output file with name config.cfg</p>
  
  <p>Thanks in advance!</p>