[cmake] Set DUMUX_MULTITHREADING_BACKEND in config.h
only set DUMUX_MULTITHREADING_BACKEND if it's not defined. We want to be able to overwrite the flag per target (as for example used for the Python bindings which enforce Serial)
only set DUMUX_MULTITHREADING_BACKEND if it's not defined. We want to be able to overwrite the flag per target (as for example used for the Python bindings which enforce Serial)