Don't depend on gl.

SDL dlopens it internally when needed, or gles for android, so get rid of the
code that used gl and don't include or link to it.
Add sdl2_mixer to android.mk
Don't try to set locale in android.

I'll have to see how to substitute that. For now, ifdef'd out.
ndk doesn't like an enum as a SDL_Keycode.
No posix_memalign for android.
Remove some more unused files/code.
Fix use of transform and delete more unused files.
Link to SDL companion libs.

I took an Android.mk for sdl_gfx from
https://github.com/dailin/wesnoth_ios/blob/master/lib/SDL_gfx/Android.mk, but
compilation fails because there are some differences between SDL and SDL2. Only
those symbols are lacking for openxcom.