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: I90c319bdda6b710bce4d2d883736716a70760127
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>