title: How to use / include libraries in Cmake / Kdevelop
body: <p>I don't understand what I need to do to use a library which is located in /usr/include. </p>
  
  <p>For example: I want to use the <a href="https://github.com/open-source-parsers/jsoncpp" rel="nofollow">json library</a> which is located in /usr/include/json. 
  In my project 'main.cpp' I do <code>#include &lt;json/json.h&gt;</code>.</p>
  
  <p>I don't get any errros but when I start to use the functions in the library I get undefined reference errors. I have this problem with multiple libraries, I have no idea what to do I searched on google but I only came across vague answers.</p>
  
  <p>I'm pretty sure I need to do something in the CMakeLists.txt file but I have no idea what. </p>