fix CLucene build for MSVC 2013

Commit qtbase/9f0e5d00ab51cc7c0dc87c8d72f48c4e6edaf120 added the
/Zc:strictStrings compiler option to MSVC 2013 builds.
We're circumventing the problem by throwing const_casts into the code.

Change-Id: I45cc647b780f834b756caed92c4d6f9a50a53664
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>