title: What does 'module' in cmake 'pkg_check_modules' stand for?
body: <p>I am building a certain module in GNU Radio. (Let's call it my-module.)  </p>
  
  <p>GNU Radio uses <code>cmake</code> to build and intall a my-module.</p>
  
  <p>In the project, I need to use a module that was made by others. This can be <code>it++</code> and other gnuradio-modules. And I will be using a gnuradio module. (Let's call it <code>other-module</code>.)</p>
  
  <p>To use other modules in my gr-module, I need to use <code>find_package</code>, <code>pkg_check_modules</code> and <code>find_library</code> to tell 'I will be using pre-built modules'<br>
  And <code>pkg_check_modules(PC_ITPP itpp)</code> can successfully detect it++.<br>
  But I am having difficulties in finding <code>other-module</code>. As I put <code>pkg_check_modules(PC_OTHER_MODULE other_module)</code>, it can't detect the module.</p>
  
  <p>What I am curious is, what does <code>module</code> in <code>pkg_check_modules(&lt;PREFIX&gt; &lt;MODULE&gt;)</code> stand for and how and where does cmake find a given module?</p>
  
  <p>Of course, I don't think it's a kernel module since I don't see it++ in the list of kernel modules. Then, are they cmake-modules? I don't think so, neither, because<code>cmake --help-module-list</code> doesn't show it++.</p>
  
  <p>Is there a special file that contains a list of modules that can be referred by cmake? If then, should <code>it++</code>and other library, modules be registered in that file at the build and installation steps in order to be recognized as modules by cmake?</p>