[3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8

[felixonmars]

Define UNUSED only when mpdecimal.h doesn't define it. This would support
building with system libmpdec 2.5 while retaining compatibility with system
libmpdec 2.4 or the vendored copy.

Tested to build fine with either system libmpdec or the vendored one.

https://bugs.python.org/issue41302

[skrah]

Thank you, this looks very safe given that neither _decimal.c nor libmpdec will change in 3.8.

Could you add a News entry to justify this? Something like:

"Enable building Python 3.8 with libmpdec-2.5.0 to ease maintenance for Linux distributions. Patch by Felix Yan."

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[felixonmars]

I have made the requested changes; please review again. (Used blurb_t for the first time, hopefully I did it right)

[bedevere-bot]

Thanks for making the requested changes!

@skrah: please review the changes made to this pull request.

[skrah]

Even though the patch is obviously correct, I've tested this with the included libmpdec, Ubuntu's libmpdec-dev-2.4.2 and libmpdec-2.5.0.