title: Using cmake options in generator expressions
body: <p>I would like to use a cmake option inside a generator expression
  in order to turn on a certain compile flag. From the <a href="https://cmake.org/cmake/help/v3.10/manual/cmake-generator-expressions.7.html" rel="noreferrer">documentation</a> it is not clear to me how to achieve this.</p>
  
  <p>I would like to write something like </p>
  
  <pre><code>option(MYOPTION " ... " OFF)
  ...
  add_compile_options($&lt;$&lt;MYOPTION&gt;:-lblas&gt;)
  </code></pre>
  
  <p>which does not work. </p>
  
  <p>Is there a way to achieve this?</p>