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

test/testcustomcursor.c

@@ -9,15 +9,16 @@
   including commercial applications, and to alter it and redistribute it
   freely.
 */
-#include <stdlib.h>
-
-#ifdef __EMSCRIPTEN__
-#include <emscripten/emscripten.h>
-#endif
 
 #include <SDL3/SDL_test_common.h>
 #include <SDL3/SDL_main.h>
 
+#ifdef SDL_PLATFORM_EMSCRIPTEN
+#include <emscripten/emscripten.h>
+#endif
+
+#include <stdlib.h>
+
 /* Stolen from the mailing list */
 /* Creates a new mouse cursor from an XPM */
 
@@ -329,7 +330,7 @@ static void loop(void)
         }
         SDL_RenderPresent(renderer);
     }
-#ifdef __EMSCRIPTEN__
+#ifdef SDL_PLATFORM_EMSCRIPTEN
     if (done) {
         emscripten_cancel_main_loop();
     }
@@ -411,7 +412,7 @@ 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) {