Fix GMP v4 compatibility.

We had started relying on GMP 5.x (for usage of mpz_powm_sec), but this
is pretty painful on RHEL-esque targets, which still use GMP 4.x.

In the mean time while we're still supporting this, it's easier to just
fallback to mpz_powm when _sec is unavailable, and emit a WARNING for
using the primitive.

This also installs a header, HsIntegerGmp.h, which clients could use for
a fallback.

As a side note, this will probably also help Debian oldstable users who
might have outdated GMP versions (which I believe is the cause for #8666.)

Reviewed-by: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Austin Seipp <austin@well-typed.com>