signal. First, selection of the GUI should cause BX_GUI_SIGHANDLER to
be defined in config.h.in. Then, the GUI should define member functions
Bit32u get_sighandler_mask ();
void sighandler (int sig);
The mask function returns a bitfield where one bit corresponds to each
signal. For any signal whose bit is set to 1 in the return value of
get_sighandler_mask, the gui will control that signal. When the signal
arrives, bx_gui.sighandler(sig) will be called by bx_signal_handler,
instead of the default behavior of that signal.
