fix "invalid subsystem" warning for MSVC

Look at the MSVC version of the mkspec Qt is built with to determine
the minimum Windows version. Also, use 5.2 as minimum Windows
version for 64 bit.

Task-number: QBS-406

Change-Id: I4693cc055eb5309e8c5528e94a03d264fc553c67
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>