title: What is the correct way to customize the install output directory for each developer in CMake?
body: <p>I've been working on an old game that I created CMake files for to get rid of a mix of Makefiles and visual studio projects. Everything is working well, but I'm having a hard time figuring out what the correct way to allow the developer to specify where the output files are copied to when install is run.</p>
  
  <p>The issue is there are many DLLs and some custom targets that need their output copied into a directory structure that includes the game data (levels, art, sound, etc) before they can test the code.</p>
  
  <p>My install commands currently uses a variable that I 'SET' at the top level CMakeLists.txt to specify the output directory. I've tried overriding it with -DD3_GAMEDIR on the cmake command line. That variable gets set in the CMakeCache, but the SET command appears to override it still.</p>
  
  <p>Should I be checking the length of the variable before using SET to see if the user specified a value? That seems like a hack to me, but I'm having a hard time finding the correct way to do it.</p>
  
  <p>Thanks!</p>