bug#32425: build fails due to undefined reference to mpz_roinit_n

[Glenn Morris]

Package: emacs
Version: 27.0.50

Since 78ec68e, builds on hydra.nixos.org fail with:

alloc.o: In function `make_pure_bignum':
  alloc.c:5570: undefined reference to `mpz_roinit_n'

A quick search suggests this function was added in gmp 6.
hydra is using gmp 5.1.3.
There is no gmp version test in configure.ac, and no option to force use
of mini-gmp.
This suggests builds on eg RHEL6 would fail in the same way.

Ref eg https://hydra.nixos.org/build/79486113