patch for speed

[paul zimmermann]

> How many "users" will want to use tune/speed and at the same time not be
> able to figure out that we alias mpz_legendre to mpz_jacobi?  That alias
> might not be documented, but tune/speed is "more undocumented".

note also that since mpz_legendre only accepts odd primes as second argument,
its running time might differ from that of mpz_jacobi, even if it is an alias.
Anyway if my contributed patch is not welcome, just ignore it!

Paul