Select Visual Studio version based on mkspec

Change 7a1e0d57

 determined the Visual Studio version based
on the 'VisualStudioVersion' environment variable. While this variable
is defined by the default Visual Studio command line prompt, it isn't
necessarily set in other environments.

Anyhow, Qt already determined the compiler/Visual Studio version in the
mkspec. We can just reuse this here.

Change-Id: Ibee9526e91ae855151940da6be438705de048c0c
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>