title: Improving cleaning of CMake build directory
body: <p>I'm trying to figure out a way to configure my CMake build to include a "clean" target that will remove all files in the build directory except for those in a subdirectory named "modules".</p>
  
  <p>Why, you may ask? Well, our codebase is currently makefile based and I'm switching it to cmake. My current setup is to add a cmake/ directory at the top level that includes required macros in the modules/ subfolder. People can then run "cmake .." from the cmake/ directory.</p>
  
  <p>Often when I make changes in my CMakeLists.txt I want to be absolutely sure none of my previous cmake runs hold influence, so it's good to start from scratch in the cmake/ directory. I can't just rm -rf ./ since there is the required modules/ directory. I want a simple "cmake clean" way of restoring it, if possible.</p>
  
  <p>Thanks for any input.</p>