Updated check for stdbool.h
If you're building in a C99 or newer environment, we'll automatically include stdbool.h.
If you're building in a C89 or C90 environment, we'll assume that stdbool.h isn't available and define bool as an unsigned char. If you have a working stdbool.h and want SDL to use it, you can either include stdbool.h before SDL headers, or define SDL_INCLUDE_STDBOOL_H in your project.
Closes https://github.com/libsdl-org/SDL/pull/10980