Update CMakeLists.txt to work on CMake 3.12
Update CMakeLists.txt to work on CMake 3.12

This fixes the following errors when trying to build with CMake 3.12:

Error copying directory from "/home/neel/git/seasocks/src/ws_chatroom" to "/home/neel/git/seasocks/src/app/c/src/ws_chatroom_web".
mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,offa/seasocks,offa/seasocks,offa/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,offa/seasocks