Starting with Visual Studio 2017 Update 9, Visual Studio doesn't like
the MinimalRebuild option anymore and warns:
cl : Command line warning D9035: option 'Gm' has been deprecated and
will be removed in a future release
The option can be safely removed so that the default is used.
Closes https://github.com/curl/curl/pull/3425
The Visual Studio builds didn't use IPv6. Add it to all projects since
Visual Studio 2008, which is verified to build via AppVeyor.
Closes https://github.com/curl/curl/pull/3137