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: I36449cfa199db8bd25ffb0f7b384346cd74897d8
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>