QApplication: use qEnvironmentVariableIntValue()

It doesn't allocate memory, so cannot throw and is a lot faster
than qgetenv().

Change-Id: I3bd7b79b520ddbb9d4dd6adb3a495e214f111c2c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qapplication.cpp

@@ -636,7 +636,7 @@ void QApplicationPrivate::initialize()
     qRegisterGuiStateMachine();
 #endif
 
-    if (qgetenv("QT_USE_NATIVE_WINDOWS").toInt() > 0)
+    if (qEnvironmentVariableIntValue("QT_USE_NATIVE_WINDOWS") > 0)
         QCoreApplication::setAttribute(Qt::AA_NativeWindows);
 
 #ifdef Q_OS_WINCE