Checkpoint: Rebuild Engine

Implement three functions: 1. int rebuild(long long tgt, const long long *deps, size_t n, int phony, int strict) — rebuilds if phony; else if strict, any prerequisite *not older* than the target triggers (>=); else only strictly newer (>). 2. size_t count_rebuilds(long long tgt, const long long *obj_mtimes, size_t n) — how many object files (as targets of their own rules) are older than the changed source at tgt... model: count objects whose mtime < tgt (they will rebuild). 3. const char *strictness_effect(int strict) — return "more rebuilds" for strict=1, "fewer rebuilds" for strict=0.