Define SDL_PLATFORM_* macros instead of underscored ones (#8875)

src/main/generic/SDL_sysmain_callbacks.c

@@ -23,7 +23,7 @@
 #include "../SDL_main_callbacks.h"
 #include "../../video/SDL_sysvideo.h"
 
-#ifndef __IOS__
+#ifndef SDL_PLATFORM_IOS
 
 static int callback_rate_increment = 0;
 
@@ -80,4 +80,4 @@ int SDL_EnterAppMainCallbacks(int argc, char* argv[], SDL_AppInit_func appinit,
     return (rc < 0) ? 1 : 0;
 }
 
-#endif // !__IOS__
+#endif // !SDL_PLATFORM_IOS

src/main/ios/SDL_sysmain_callbacks.m

@@ -22,7 +22,7 @@
 #include "SDL_internal.h"
 #include "../SDL_main_callbacks.h"
 
-#ifdef __IOS__
+#ifdef SDL_PLATFORM_IOS
 
 #import <UIKit/UIKit.h>