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
I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```
I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
else
find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
fi
done
```
Fixes https://github.com/libsdl-org/SDL/issues/6575
