CI: Allow to re-use findings cached by earlier `configure` script runs

[jimklimov]

Currently all local and CI builds safely default to not using cached findings about method availabilities, etc.

We do however suffer long build times in general, and prohibitively long on AppVeyor and sometimes CircleCI (time-limited VMs) in particular.

It was checked locally that running with different configure options and trusting older config.cache is counter-productive (e.g. libusb 1.0 vs 0.1 method availability records are misleading). However, for repetitive runs of same configuration in same environment this feature can be helpful.

• Detecting configuration/packaging changes vs. a previous run, in order to invalidate an old config.cache file when it is obsolete, could be a good bonus feature.
• Alternately, add optional caching-or-not to the autotools methods used in our m4 sub-scripts for specific dependencies. so some builders can re-evaluate those items always and others would use them from cache, while standard libc method availability there etc. would be always cached if possible.