title: Set solution folder property in CMake for 3rd-party library
body: <p>My project depends on a large 3rd party library (OpenMVG).
  Currently it is not installed. Official building instructions recommend use it with <code>add_subdirectory</code>.</p>
  
  <p>Therefore my <code>CMakeLists.txt</code> contain the following lines</p>
  
  <pre><code>add_subdirectory(${OpenMVG_DIR}/src "${CMAKE_BINARY_DIR}/openMVG")
  include_directories(${OpenMVG_INCLUDES})
  add_executable(f_matching f_matching.cpp)
  target_link_libraries(f_matching ${OpenMVG_LIBS})
  </code></pre>
  
  <p>My platform is the Visual Studio, and generated solution contains 72 projects on the top level. I'd like to use <code>FOLDER</code> CMake property to organize it in hierarchical levels. For example, I'd like to move all OpenMVG's projects to the separate VS solution folder.</p>
  
  <p>However, as the CMake documentation says, this property can be only assigned to targets, not to directories.
  My <code>CMakeLists.txt</code> contains </p>
  
  <pre><code>set_property(DIRECTORY ${OpenMVG_DIR}/src PROPERTY FOLDER OpenMVG)
  </code></pre>
  
  <p>but this line does nothing, no solution folders appear.
  I've tried to modify a couple of OpenMVG's CMakeLists by setting <code>FOLDER</code> property on targets, this worked.</p>
  
  <p>Is there any workaround for this situation, without changing CMakeLists from OpenMVG?
  Currently, I'd like to spend my time for other tasks, and postpone this simple editing for future.</p>