CI: we would benefit from a regular job building against latest third-party code

[jimklimov]

As rediscovered recently during investigation for #1289 or #1332, as well as many other times earlier, third party code NUT depends on does evolve :)

It would be beneficial to became pro-active rather than scrambling reactively (e.g. as with large autoconf changes with their new release after a several years hiatus), and set up a regular (weekly?) job to build as much of the ecosystem as we can from source, using Git HEADs of the components involved (net-snmp, libusb, countless others), on at least one platform.

Leaning on works I've done before -- since NUT CI scripting was already inspired by and cross-pollinated with zeromq/zproject work in the area, their approach to fallback-building the world in one place can be useful here as well. Alternately, the 42ity/FTY "dispatcher repo" reaches similar goals a bit differently, and may be more useful for parallelized and multi-platformed builds.

NOTE: A rough stub is available in scripts/Windows resources (README and shell scripts) where I documented, and drafted a bit of automation for, building prerequisites missing in mingw packaging. This could be extended for the task.