[Development] Prebuilt 32-bit versjon for MSVC 2017

[Tim Blechmann]

> Currently Qt ships with 32-bit binaries for MSVC 2015, but not for
> MSVC 2017. The Qt Company have received requests for adding prebuilt
> 32-bit binaries also for MSVC 2017. By adding more prebuilt binaries,
> we increase the complexity, and add to the overall maintenance of our
> QA system. Our preferred approach would be to add binaries for MSVC
> 2017 and remove the binaries for MSVC 2015, as the numbers of
> binaries in the build remains the same. It will still be possible to
> manually build Qt for MSVC 2015, in the same way as it is possible to
> manually build for MSVC 2017 today. Feedback and viewpoints are much
> appreciated and will enable us to make the best possible decision.

the ABI of msvc2015 and msvc2017 is the same. so unlike earlier msvc
releases, one can mix binaries from both compilers.

in theory. in practice there is a still unresolved compiler bug in
msvc2017 (we've experienced it only with static libs and debug builds,
but i'm not sure if it has any implication on the binaries you want to
release):

https://developercommunity.visualstudio.com/content/problem/76198/vs-2017-compiler-creates-broken-debug-build-using.html

cheers,
tim