save QT_CONFIG in Qt.core.qtConfig

We want to access the QT_CONFIG from Qt modules.
The property will be used e.g. for determining whether
we're dealing with an ANGLE build.

Change-Id: Ic5cafc23c438c06e3b25ee73eca68c25aefc97ea
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>