title: CMAKE: target_include_directories and include_directories
body: <p>I'm reading the cmakelist file of a big project, and confused about the usage of <code>target_include_directories</code> and <code>include_directories</code>.</p>
  
  <p>The <code>include_directories</code> should be able to add all header files to path where the compiler search for. But I still see some <code>target_include_directories</code> in the cmakelist file of sub directories, that specify some include path for a specific target.  </p>
  
  <p>Can anybody explain the usage of these two? As far as I concerned, add all paths of header file to <code>include_directories</code> should be enough.</p>