winbuild: build with warning level 4

This is consistent with 7bc64561, which
changed the warning level from 3 to 4 for the Visual Studio project
files. But disable the level 4 warning C4127 "conditional expression is
constant", as that one is issued by older versions of the Windows SDK
as well as curl itself under some circumstances.

Closes https://github.com/curl/curl/pull/1667