Allow use of MPIR instead of GMP

[yperbasis]

GMP is not compatible with Windows, while its fork MPIR is.

[ValarDragon]

This seems like a good change to me, but I haven't looked into MPIR in detail yet.

I don't have convenient access to a windows computer at the moment, does the library build on windows with this?

Is building on Windows something you've had to look into @AntoineRondelet ?

[yperbasis]

MPIR is a fork of GMP and is also available on Linux/macOS. I can build libff fine on Windows with MPIR. There was one issue due to the fact that long is 32 bit on Windows even in the 64-bit mode (unlike Linux). I've raised PR #59 to fix that.

[AntoineRondelet]

Is building on Windows something you've had to look into @AntoineRondelet ?

No, unfortunately I haven't looked into that, and am not knowledgeable (at all) about Windows.

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Thanks @yperbasis for the PR. I would keep this PR open for the time being as the CI is not comprehensive enough to safely support multiple OSes and architectures. As such, I would be in favor to first: Add windows build to the CI (e.g. https://docs.travis-ci.com/user/reference/windows/ or https://github.com/actions/virtual-environments/blob/main/images/win/Windows2019-Readme.md), and then (and only then) consider merging Windows related PRs.

Note that, macOS is not part of the CI either. We need to add that.
I'll open a ticket to extend the CI [EDIT:] Here's the ticket #64