title: What is meaning of 'set(CMAKE_REQUIRED_LIBRARIES "m")' in CMake Tutorial?
body: <p>I am learning CMake with <a href="https://cmake.org/cmake/help/latest/guide/tutorial/index.html" rel="nofollow noreferrer">CMake Tutorial</a> and found something which is not clear for me:</p>
  
  <pre><code>include(CheckSymbolExists)
  set(CMAKE_REQUIRED_LIBRARIES "m")
  </code></pre>
  
  <p>So what is the <code>CheckSymbolExists</code>? Is it a function or a lib?
  What's meaning of the "m"? Does it mean a lib name or some flag?</p>
  
  <p>I had tried to read through cmake documents, but I just don't understand.
  Please somebody help me to understand these.</p>