Prefer GMP's multiplication routine (if GMP version >= 4.0).

Prefer GMP's multiplication routine (if GMP version >= 4.0).

Recent GMP multiplication routines (mpn_mul) are somewhat faster than CLN's.
For huge operands, this is due to better tuned FFT. For medium-sized operands
it's due to more algorithms (Toom 3-way.) For small operands, I couldn't find
a difference. Let's just use mpn_mul if it's available.

For huge binsplit sums, this can half the runtimes, on amd64.