Use QT_CONFIG to check if QtJson is available.
Use QT_CONFIG to check if QtJson is available.

Change-Id: I8dd1c67d6e75a023a6d2ebf673644799eff58f55
Reviewed-on: http://codereview.qt.nokia.com/3063
Reviewed-by: Qt Sanity Bot <5581206bb7e0307f0d99eb71898ae6b694149ca5@ovi.com>
Reviewed-by: Xizhi Zhu <6095b3dbb866d5a6b4a607e0cbd4aaa65e13c0b2@nokia.com>

qtproject/qtsystems,qtproject/qtsystems