Update SDL to 2.0.18

libs/SDL2/test/testloadso.c

@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 1997-2020 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2021 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages
@@ -44,7 +44,7 @@ main(int argc, char *argv[])
         return 2;
     }
 
-    if (strcmp(argv[1], "--hello") == 0) {
+    if (SDL_strcmp(argv[1], "--hello") == 0) {
         hello = 1;
         libname = argv[2];
         symname = "puts";