use -Wextra instead of -W for gcc toolchain
use -Wextra instead of -W for gcc toolchain

-W is deprecated for ages.

Task-number: QBS-149

Change-Id: Id179d59824f6e6a392b967ecd244e8697ae1f529
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@digia.com>

Distrotech/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,villytiger/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,villytiger/qbs,qt-labs/qbs,Distrotech/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,Distrotech/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs