Added SDL_srand(), SDL_rand(), and SDL_rand_r() (thanks @JKaniarz!)

These are simple random functions that should not be used for serious random number generation.

Fixes https://github.com/libsdl-org/SDL/issues/4968

test/testpopup.c

@@ -20,7 +20,6 @@ freely.
 #endif
 
 #include <stdlib.h>
-#include <time.h>
 
 #define MENU_WIDTH  120
 #define MENU_HEIGHT 300