bsd.qt.mk: Use CONFIGURE_WRKSRC instead of WRKSRC in the .qmake.cache hack

WRKSRC is not what we want here: when USES=qmake:outsource is used (such as
in www/webkit-qt5), the build actually takes place in ${WRKDIR}/.build, so
we were creating .qmake.cache in the wrong location and passing the wrong
directory to the linker via -L. With CONFIGURE_WRKSRC, we get the right
value regardless of whether :outsource is used or not.

PR: 212859
Reviewed by: tcberner
MFH: 2016Q3