The IPolicyConfig file is a third party file and since it describes a
third party API we can't make changes to it. Adding it via the system
includes disables warnings on most compilers.
Encapsulating compiler specific options in a separate file
makes it easier to reason about what is going to happen on
a specific qmake call.
clang-msvc and msvc are separate because they don't support
exactly the same compiler switches.
Vasily