[CM] s7: Whether to use WITH_GMP

[bil at ccrma.Stanford.EDU]

In the gmp version, it's probably safest to assume
everything is going through gmp. In cases where the
optimizer can ascertain ranges, s7 will use
faster code, but I just tore out half the optimizer,
and am rebuilding it.  So I don't know how the
in-between stuff interacts with gmp -- give me a
month or two!  My old timings indicated that gmp
could be 10 to 50 times slower than straight C (but at
least it gave the right answer).