Overriding headers has always been risky, and finally we see why.
Apparently on iOS, files may be compiled in such a way that the original
SDL_config.h file is included, not Lime's custom one. It's hard to do
anything more than we've already done to manage include order, so we
need to find another option.
One such option is to make the two files as similar as possible, such
that it usually doesn't matter which one ends up being included. Then we
only set `USING_GENERATED_CONFIG_H` when it really matters.
