Policy-class design: how to add a CMake option selecting the desired class

Question

I wrote the following traits that defines some policy classes:

  namespace memory
  {
    struct on_RAM
    {
      template<typename T>
      using pop_sizes_type = PopulationSizeOnRAMImplementation<T>;

      template<typename T>
      using flow_type = FlowOnRAMImplementation<T>;
    };

    struct on_disk
    {
      template<typename T>
      using pop_sizes_type = PopulationSizeOnDiskImplementation<T>;

      template<typename T>
      using flow_type = FlowOnDiskImplementation<T>;
    };
  } // end namespace memory

I use these traits to configure the behavior of a simulation class: MyClass<memory::on_RAM> or MyClass<memory::on_disk>

Eveything works fine, but I would like users of the resulting programs to be able to pick on or the other alternative without having to modify the source code.

I guess I can ask them to overwrite a default implementation by giving an option to CMake, that could give that to the compiler, that could make some compile-time choice. But I have no idea how to technically implement that. Any suggestion?