In the .etk file:

widget
{
   type: "button";
   name: "custom-signal-button";
   parent: "button-window-button-frame";
   label: "custom signal";
   signal
   {
      name: "clicked";
      emit: "custom_signal_emittion";
   }
}

In the .c file:

void _custom_signal_emittion_cb(void *data)
{
   printf("This is a custom emittion!\n");
}

...
...
evolve_signal_connect(evolve, "custom_signal_emittion", _custom_signal_emittion_cb, NULL);