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: Ib46b9e83f690b0b26933b73764c7d7d4b031d792
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>