Add `HAVE_STDBOOL_H` define for Alberta when `stdbool.h` is available

In older versions of Alberta, it tries to fix the issues around not having proper bool in C by redefining bool as _Bool. Clang's stdbool header defines _Bool as bool, which then leads to a typedef bool bool and that gives a compile error. This whole bool/_Bool logic is inside an ifdef in Alberta that is only active if HAVE_STDBOOL_H is not defined. If HAVE_STDBOOL_H is defined, however, it just includes stdbool.h and uses that. So we can check during CMake configuration if stdbool.h is available and set the flag accordingly.