Changes to integer-gmp

Changes to integer-gmp:
commit d7bff4dddfa10389156ca11f75a5a23e78cf3ab0
Author: Austin Seipp <austin@well-typed.com>
Date:   Wed Feb 26 16:18:20 2014 -0600

    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    might have outdated GMP versions (which I believe is the cause for #8666.)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>