- Allow custom SDL GetPlatform name with env var SDL_PLATFORM

Index: src/SDL.c
--- src/SDL.c.orig
+++ src/SDL.c
@@ -544,6 +544,10 @@ int SDL_GetRevisionNumber(void)
 /* Get the name of the platform */
 const char *SDL_GetPlatform(void)
 {
+    const char *platform;
+    if (platform = SDL_getenv("SDL_PLATFORM")) {
+        return platform;
+    }
 #if __AIX__
     return "AIX";
 #elif __ANDROID__