Building NUT for CI in Windows workers

[jimklimov]

The "fightwarn" effort started in #823 introduced in particular builds of NUT in Windows workers, currently for the sake of seeing if we can do it at all. This can be seen in branches with "fightwarn" in the name, and possibly truncated in scope (to avoid running other test cases while fixing this scenario) by a temporary patch like these from the https://github.com/networkupstools/nut/commits/fightwarn-win branch:

• 44587b0 - [TEST] .travis.yml: in this branch only build windows gnu
• 4f52196 - .travis.yml: constrain Windows builds to branches whose names contain "fightwarn-win" specifically, until someone figures out how to make autotools work

As the latter comment says, the current blocker in this path is the lack of functional autotools which would actually generate the configure script (probably as a .bat or .cmd file, though a posix shell script could work too for that environment).

[jimklimov]

Bookmarking a couple more findings: apparently Microsoft itself now issues (temporarily valid) VMs that can be used for building and/or testing stuff. This may be useful even when NUT CI life continues beyond Travis.

The list found so far includes:

• https://developer.microsoft.com/en-us/windows/downloads/virtual-machines/ - a development VM with Visual Studio, WSL and Ubuntu; validity limited by calendar date; I assume by that time a newer version of such VM would be published so developers test on freshest rolling platform.
• https://developer.microsoft.com/en-us/microsoft-edge/tools/vms/ - a series of VMs intended for browser testing (matrix of Windows and browser versions) limited by 90 days from first boot
• https://aka.ms/windowsdevelopervirtualmachineeula - rules of engagement (reading needed)

[jimklimov]

Seems CircleCI offers up to 6000 minutes per month (to be explored for MacOS per #1419) including Windows builders:

• https://circleci.com/docs/2.0/hello-world-windows/
• https://circleci.com/docs/2.0/configuration-reference/#docker-machine-macos-windows-executor
• https://circleci.com/docs/2.0/executor-types/

[jimklimov]

Getting closer with:

• CI integration of NUT for Windows builds - native Windows + MSYS2 #1552 (to be merged soon)
• Provide more prereqs for native Windows MSYS2 builds (snmp, ltdl notably missing) #1475 (needs more work ATM)

Maybe sometime in the future would get to the likes of:

• Investigate native Windows mingw builds (without MSYS2, Cygwin, etc.) #1476
• Investigate fully-native Windows builds (with Visual Studio compilers and tools) #1490
• CI integration of NUT for Windows builds - go (semi-)native with LLVM-MINGW provided clang on Windows #1647

[jimklimov]

Initial milestone achieved (we have some way to build and test NUT in Windows). Other goals are worthy nice-to-haves to expand the supported base of options later, so not closing this issue yet but not actively pursuing either.