title: Populating ${SRCS} from CMakeLists.txt in subdirectories
body: <p>I currently define a <code>${SRCS}</code> variable in the <code>CMakeLists.txt</code> file in my projects root directory, listing every source file required for my executable:</p>
  
  <pre><code>SET (SRCS main.cpp
            dir1/file1.cpp
            dir1/file2.cpp
            dir2/file3.cpp
            dir2/file4.cpp)
  </code></pre>
  
  <p>How can I distribute this list across the <code>CMakeLists.txt</code> files in each of the subdirectories? That is:</p>
  
  <pre><code>CMakeLists.txt
      Adds main.cpp to SRCS and subdirectories dir1 and dir2
  
  dir1/CMakeLists.txt
      Adds file1.cpp, file2.cpp to SRCS
  
  dir2/CMakeLists.txt
      Adds file3.cpp, file4.cpp to SRCS
  </code></pre>