Windows Phone qmake: look up MSVC in the registry

We were checking the path for cl.exe, which could be any MSVC. Instead,
look up the path from the registry and find the SDK from there.

Squash with d8bfdc and friends.

Change-Id: Ieb6f7d0bb20faed437d0ec06a1b5c186b1d5e02d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>