android sdl static link

Follow the readme of android, it should work, but there is two tricks I have to mention.

1. You can put SDL folder anywhere but not in jni, otherwise you will get redefine error of NDK as "Trying to define local module".
2. You have to define main as "int main(int argc, char *argv[])".  "char **argv" will not work, and leads to undefined SDL_main linking error.