Added an SDL_FORCE_INLINE macro.
Improvements to stdlib.

All SDL_* functions are always available as real symbols, so you can always
link against them as a stable ABI. By default, however, all the things that
might have dithered down to macros in your application are now force-inlined,
to give you the same effect as before and theoretically better performance,
but still solve the classic macro problems.

Elsewhere, we provide real functions for these things that simply wrap the
inline functions, in case one needs to have a real function available.

Also: this exposed bugs: SDL_abs() does something different if you had the
macro vs the libc function, SDL_memcpy() returns a void* in the function
but not the macro, etc.
Replace all the "static __inline__" functions with SDL_FORCE_INLINE.