add compiler info to --version

[geekosaur]

I'm not sure if this counts as a significant change or not, since it's possible scripts depend on the output. (The git info change doesn't really count, since it's only operable on unreleased versions.)

hilfy «cabal+more-version-info¤@a11e10471» Z$ $(cabal list-bin cabal) --version
Warning: this is a debug build of cabal-install with assertions enabled.
cabal-install version 3.17.0.0 (commit a11e104 on more-version-info, Thu Dec 11 16:57:14 2025 +0000)
compiled using version 3.17.0.0 of the Cabal library (commit de16351, Thu Nov 27 00:06:37 2025 +0000)
with ghc 9.10.3 on linux x86_64

The information added by this change is the last line. If a development GHC version is used to build cabal, the version will include a datestamp (e.g. 9.14.0.20251128).

Template Α: This PR modifies behaviour or interface

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
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[geekosaur]

This is a proposed change, and open to review and suggestions.

[Mikolaj]

LGTM

[ulysses4ever]

Can you show how it'll look like in the description? Also, it'd be good to add some motivation. Remember that by default we expect a ticket before a PR.

[geekosaur]

The motivation was trying to get that information from someone on Discord who was having odd problems with cabal, and IIRC it turned out they were using a cross-built cabal.

I don't consider this final by any means; some programs have a --version-full option, for example, which includes this kind of information (and the GIT_REV stuff might also move there if we add it).

[geekosaur]

I moved that #if to a new Distribution.Client.Compat.SysInfo module.