Workaround webkit deadlock on macos x

The webkit AtomicallyInitializedStatic and Qt's Q_GLOBAL_STATIC can
deadlock on the Mac, as the mac compiler inserts calls to
__cxa_guard_acquire and __cxa_guard_release around initialisation of
local statics.
In Q_GLOBAL_STATIC case, this is the QGlobalStaticDeleter local static
Whereas webkit AtomicallyInitializedStatic is a local static variable
in any case.

Problem is triggered because webkit constructs QNetworkConfigurationManager
inside the constructor of a local static - networkStateNotifier
And the generic bearer plugin calls QNetworkInterface::allInterfaces
in the bearer thread, which needs an initialised Q_GLOBAL_STATIC.

Reviewed-by: Laszlo Agocs
(cherry picked from commit 38db40d9a2db44e47b0aabd9487284cd1106b353)
Task-number: QTBUG-24554

Change-Id: Id2c7818faefb46e66b0bbc30ce30595d46d53016
Reviewed-by: Zeno Albisser <zeno.albisser@nokia.com>

src/plugins/bearer/generic/qgenericengine.cpp

@@ -147,6 +147,9 @@ static QNetworkConfiguration::BearerType qGetInterfaceType(const QString &interf
 QGenericEngine::QGenericEngine(QObject *parent)
 :   QBearerEngineImpl(parent)
 {
+    //workaround for deadlock in __cxa_guard_acquire with webkit on macos x
+    //initialise the Q_GLOBAL_STATIC in same thread as the AtomicallyInitializedStatic
+    (void)QNetworkInterface::interfaceFromIndex(0);
 }
 
 QGenericEngine::~QGenericEngine()