Update THESHOLD name in jacobi.c

[Niels Möller]

Seth Troisi <braintwo at gmail.com> writes:

> Per Niels' suggestion I'm taking a look at the Jacobi documentation.

Algorithm description was most recently posted here:
https://gmplib.org/list-archives/gmp-bugs/2017-August/004190.html

> I noticed that jacobi.c aliases GCD_DC_THRESHOLD then uses both names once.
> https://gmplib.org/repo/gmp/file/tip/mpn/generic/jacobi.c#l177
> https://gmplib.org/repo/gmp/file/tip/mpn/generic/jacobi.c#l196

I agree it should be JACOBI_DC_THRESHOLD at both places. This makes it
possible (but optional) to use a separaately tuned jacobi threshold.
Which is unlikely to make much difference. The use of mpn_hgcd2_jacobi
rather than mpn_hgcd2 implies a *sligthly* larger constant for the linear
part of the work.

Regards,
/Niels

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