configure.php: prepare PhD auxiliary-file generation for move to php/phd

[lacatoire]

Prepares moving PhD auxiliary-file generation (version.xml, sources.xml, fileModHistory.php) out to php/phd, without removing anything yet so this can land independently.

• Add phd_conf_json(), which serializes the build state into doc-base/temp/phd-conf.json as a hand-off consumed by php/phd.
• Fix the --history-file option, which set SOURCES_FILE instead of HISTORY_FILE.
• Keep the existing phd_acronym()/php_history()/phd_sources()/phd_version() functions for now; their removal (and the matching .gitignore cleanup) depends on Add genphdfiles.php: generate PhD auxiliary files from doc-base phd#261.

Supersedes the previous full-removal approach on this branch.

[alfsb]

Wow! That is exactly what I was thinking of. Paint me very impressed.

The moved files are mentioned in .gitignore, so they can also be removed here. My long term plan is to shrink this file to only temp/.

[alfsb]

I think the doc-base side is ok. You can merge here at your earliest convenience, and yet, let this open a few days, for comments and/or reviews. I will do a heads up no Discord, to be sure.

[alfsb]

If the PhD side goes without indications of reviews or merging, then I will change this side, keep the hand-off file and PhD functions and merge in one week's time. These are some performance and doc-ext changes that are easier to do without worrying of git conflicts risk.

[alfsb]

As PhD side shows no sign of being merged, I tried to checkout, rebase and push directly into this PR, but failed. @lacatoire , please consider rebasing and hard reseting this into master HEAD, and then applying the changes of https://github.com/alfsb/doc-base/tree/pr318.

[alfsb]

Thanks for the update. Merged it as is, so as to avoid more conflicts in future, and to allow the PhD side to be developed independently.