Fixed bug 2812 - Make libSDL2main.a usable on Android via a dummy symbol

Jonas Kulla

This eliminates the need to manually compile in SDL_main_android.c.
Instead, add "-lSDL2main -Wl,-u,SDL_main_dummy" when linking.

I don't know how the nkd-build process works, but unless it was
for some reason linking libSDL2main.a it should be unaffected.

 Platforms

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 We maintain the list of supported platforms on our wiki now, and how to

 build and install SDL for those platforms:

     https://wiki.libsdl.org/Installation