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: Id3dcd3474504aebeee16dc41cd1eeab7ac21b319
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>