gh-115119: Switch Windows build to mpdecimal external

[zware]

• Issue: Unvendor libmpdec sources #115119

[zware]

I missed that mpdecimal.h does not actually exist in the source distribution, but is created (or copied) by the building of the library. This will need to be handled somehow, either by adding a Windows-specific shim that picks the right mpdecimal{32,64}vc.h to include somewhere, or by building the library separately using its own build scripts.

[zware]

Made it back around to this and fixed the build issue. I don't like the added Modules/_decimal/windows/mpdecimal.h as a permanent solution, but I think it's a decent stopgap to avoid changing the sources somehow. Ideally we'll eventually switch to using mpdecimal's own PGO build script to produce binaries, but that will be a bigger change that we can make happen later. It will also require some attention on ARM, unless 4.0.0 already has handling for that.

[zware]

!buildbot Windows

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @zware for commit c128618 🤖

The command will test the builders whose names match following regular expression: Windows

The builders matched are:

• AMD64 Windows11 Bigmem PR
• AMD64 Windows11 Refleaks PR
• ARM64 Windows Non-Debug PR
• AMD64 Windows10 PR
• ARM64 Windows PR
• AMD64 Windows11 Non-Debug PR
• AMD64 Windows Server 2022 NoGIL PR

[zooba]

@zware This looks like it's good enough shape - any reason to not merge yet?

[zware]

The limitation of 24h/day, mostly :)