Replace win32-g++ with mingw scope

Commit 773dd01 in qtbase introduced a general mingw platform scope, which
is cleaner and more flexible than matching the spec name.

(see 278152fff for a similar commit in qtbase).

Change-Id: If232fdbaacc9ce6a16c269b4c06b81efa196c2d8
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>