title: CMake: How to pass preprocessor macros
body: <p>How can I pass a macro to the preprocessor? For example, if I want to compile some part of my code because a user wants to compile unit test, I would do this:</p>
  
  <pre><code>#ifdef _COMPILE_UNIT_TESTS_
      BLA BLA
  #endif //_COMPILE_UNIT_TESTS_
  </code></pre>
  
  <p>Now I need to pass this value from CMake to the preprocessor. Setting a variable doesn't work, so how can I accomplish this?</p>