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

test/testspriteminimal.c

@@ -11,15 +11,15 @@
 */
 /* Simple program:  Move N sprites around on the screen as fast as possible */
 
-#include <stdlib.h>
-#include <time.h>
+#include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
-#ifdef __EMSCRIPTEN__
+#ifdef SDL_PLATFORM_EMSCRIPTEN
 #include <emscripten/emscripten.h>
 #endif
 
-#include <SDL3/SDL.h>
-#include <SDL3/SDL_main.h>
+#include <stdlib.h>
+#include <time.h>
 
 #include "icon.h"
 
@@ -100,7 +100,7 @@ static void loop(void)
         }
     }
     MoveSprites();
-#ifdef __EMSCRIPTEN__
+#ifdef SDL_PLATFORM_EMSCRIPTEN
     if (done) {
         emscripten_cancel_main_loop();
     }
@@ -157,7 +157,7 @@ int main(int argc, char *argv[])
     /* Main render loop */
     done = 0;
 
-#ifdef __EMSCRIPTEN__
+#ifdef SDL_PLATFORM_EMSCRIPTEN
     emscripten_set_main_loop(loop, 0, 1);
 #else
     while (!done) {