title: CMake: add all files on current directory and subdirectories with target_sources()
body: <p>In our C++ project, we have several CMakeLists.txt files (on different directories) listing every single cpp file desired with <code>target_sources()</code>. </p>
  
  <p>For example:</p>
  
  <pre><code>target_sources(&lt;Project&gt; PUBLIC
      foo_1.cpp
      foo_2.cpp
      foo_3.cpp
  )
  </code></pre>
  
  <p>This is fine in case we have few source files, but it's becoming harder for those directories where we have multiple cpp files to be added. </p>
  
  <p>Is there a simple way to tell <code>target_resources()</code> to add all files on directory and subdirectories where CMakeList.txt file is?</p>
  
  <p>For example, placing a CMakeLists.txt on a directory with multiple files and sub-directories (with more files) that just adds everything contained there. </p>