Some more macOS fixes...

1) Use static SDL since it's required by both conio and XPDev
2) Since the SDL thread will always run, any attempt to shut down
   the SDL thread calls exit(0).  Not exactly what we want to
   return, but at least this makes it not hang waiting for the
   impossible.

src/build/Common.gmake

@@ -461,6 +461,7 @@ ifndef WITHOUT_SDL
  USE_SDL	:=	1
  ifeq ($(os),darwin)
   USE_SDL_AUDIO := 1
+  STATIC_SDL := 1
  endif
 endif
 

src/conio/sdl_con.c

@@ -296,6 +296,8 @@ void exit_sdl_con(void)
 	ciolib_reaper = 0;
 	if (sdl_init_good)
 		sdl_user_func_ret(SDL_USEREVENT_QUIT);
+	else
+		exit(0);
 }
 
 void sdl_copytext(const char *text, size_t buflen)
@@ -1024,8 +1026,12 @@ void sdl_video_event_thread(void *data)
 				 */
 				if (ciolib_reaper)
 					sdl_user_func(SDL_USEREVENT_QUIT);
-				else
-					sdl_add_key(CIO_KEY_QUIT, &cvstat);
+				else {
+					if (sdl_init_good)
+						sdl_add_key(CIO_KEY_QUIT, &cvstat);
+					else
+						exit(0);
+				}
 				break;
 			case SDL_WINDOWEVENT:
 				switch(ev.window.event) {