>>77
#define isOverflow(x, y) (((x) + (y)) < (x))
だけでいいね

>>74は
#define isOverflow(x, y) (((x) >> 1) + ((y) >> 1) + ((((x) & 1) + ((y) & 1)) >> 1) >= (1 << (sizeof(unsigned int) * 8 - 1)))
かな