title: Add Source in a subdirectory to a cmake project
body: <p>I have project which has not been divided into libraries, but the source is organized in a directory tree.  I do not know how to tell cmake to go down a directory, then add the source in that directory to project defined in the parent directory.  I have attempted the following:</p>
  
  <p>in project/source/CMakelists.txt:</p>
  
  <pre><code>set(SOURCE
      ${CMAKE_CURRENT_SOURCE_DIR}/unitTest/main.cpp
    )
  add_subdirectory("${PROJECT_SOURCE_DIR}/folder1")
  add_executable(UnitTestRNG ${SOURCE} ${HEADERS})
  </code></pre>
  
  <p>then in project/source/folder1/CMakeLists.txt:</p>
  
  <pre><code>set(SOURCE
     ${SOURCE}
     ${CMAKE_CURRENT_SOURCE_DIR}/file1.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/file2.cpp
  )
  set(HEADERS
     ${HEADERS}
     ${CMAKE_CURRENT_SOURCE_DIR}/file1.hpp
     ${CMAKE_CURRENT_SOURCE_DIR}/file2.hpp
  )
  </code></pre>
  
  <p>using some message() statements, I have found that the the child folder will get the contents of the SOURCE variable, but it's new assignment to that variable will not persist on returning to the parent CMakeLists.txt</p>
  
  <p>Looking for examples and at the cmake tutorial has led me to the conclusion that:
   - Source file structures are usually flat within a project
   - If code is divided into folders, it is usually is divided into corresponding libraries.</p>
  
  <p>I wonder if there is some "best practice" from which I am deviating by attempting this structure.</p>