CMakeLists: Use aux_source_directory() instead of manually adding source files

CMakeLists: Use aux_source_directory() instead of manually adding source files
    
    Advantages:
     - No need to always manually update CMakeList.txt when sources files
       are added/removed.
     - Don't have to send a pull request every time I just fucking added
       source files to CMakeLists.txt
    
    Caveat:
    
    One has to rerun CMake every time a source file is added or removed.