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: I430ab289af99e96cd6ed6edfd683258defcb13a4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>