Webkit must not require -DQT_DLL

Apparently webkit was excluded from the actions resulting from http://thread.gmane.org/gmane.comp.lib.qt.devel/3196

This means that when using Qt5WebKitWidgets with CMake, -DQT_DLL must be defined which is unobvious and should definitely not be needed. While that could be added to the cmake file, that would be a workaround for the problem. The solution must be to make qtwebkit not require -DQT_DLL.