This adds the functionality to build Qt with clang under Windows against
the Microsoft Visual Studio 2015 runtime.

In order to replicate this, a Clang 3.8 build with Visual Studio 2015
Update 1 is needed.

Adds compiler detection to Qt to distinguish correctly the clang compiler
and Windows with Visual Studio.

Clang has some built-in numeric functions, there is no need to use the
Microsoft versions, which also conflict here.

Task-number: QTBUG-50804
Change-Id: Ia4b267a298310ac7d73edf473b12792991249d8a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>