This prime number generator works entirely like you'd expect - you put in
a number and it generates all primes strictly less than the number. So, if you
put in 5 you will get 2 and 3. It uses the Sieve of Eratosthenes to do the
computation, in a second thread (however the thread doesn't really do anything
useful, as per the exercise description.) It prints out the prime factorization
of a nonprime integer as proof that it is not prime, but you can disable this
by editing the make file - it is a compile time flag.