title: Cmake generator expressions
body: <p>I'm trying for long time to understand the benefit of <a href="https://cmake.org/cmake/help/v3.2/manual/cmake-generator-expressions.7.html" rel="noreferrer">generator expressions</a>  such as <code>$&lt;xxx:yy&gt;</code> in CMake, when and how  to use them.
  Can anybody explain it clearly with some examples.
  Many thank in advance</p>