CI: Optimize use of autoconf caching

[jimklimov]

Follow-up to PR #3109 for issue #3108, which added ways to cache autoconf discovery/configuration results (among other achievements).

While that PR did address the subject area in broad strokes, cached rebuilds on some systems did complain about effective loss of some configuration nuances, because thise values were set when running cacheable logic - and so never tickled in cached runs. This is effectively due to an old mis-use of those autotools methods (which are documented to require that nothing consequential happens in their optionally-called code).

This PR aims to make rebuilds with same config reproducible (in terms of populating same definitions in config.log, config.h, etc.) by more diligently coding the population vs. use of cached data. As a significant starting point seen as a bottleneck in eyeball profiling, probably related to numerous calls to pre-processor, compiler or linker, various m4/nut_check_lib*.m4 files were revised for this PR.

Further improvements may be possible around:

• boolean support (probably to be done as part of Refactor code from custom bool_t to stdbool.h (where available) #1176 / Consolidate bool_t definitions into a common header file #31 which may remove much of this code),
• documentation generation ability testing (e.g. a2x calls for man pages),
• systemd unit file syntax support checks,
• compiler tests seem to pause in the beginning (pragmas are cached and go fast on re-runs),
• cppunit support,
• investigate the long pause between "egrep support" and "CONFIG_FLAGS" report (first 3 or so lines of output) - likely this is where configure loads and parses the cache, but maybe there is something we do that can save some time (e.g. several calls to gitlog2version.sh may happen there and maybe can be revised into one run and then populating the data points from cache?

Also some typos and bugs were noted and addressed.

[github-actions]

A ZIP file with standard source tarball and another tarball with pre-built docs for commit c048c3c is temporarily available: NUT-tarballs-PR-3428.zip.

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