Changed the way the emulator is shut down, fixes #1555.

src/include/86box/86box.h

@@ -189,6 +189,8 @@ extern void	resub_cycles(int old_cycles);
 extern double	isa_timing;
 extern int	io_delay, framecountx;
 
+extern volatile int	cpu_thread_run;
+
 #ifdef __cplusplus
 }
 #endif

src/include/86box/plat.h

@@ -64,8 +64,8 @@ extern "C" {
 /* Global variables residing in the platform module. */
 extern int	dopause,			/* system is paused */
 		doresize,			/* screen resize requested */
-		is_quit,				/* system exit requested */
 		mouse_capture;			/* mouse is captured in app */
+extern volatile int	is_quit;				/* system exit requested */
 
 #ifdef MTR_ENABLED
 extern int tracing_on;