merge QMAKE_LIBDIR into QMAKE_LIBS early

merge QMAKE_LIBDIR into QMAKE_LIBS early

dealing with the directories separately doesn't buy us anything. it's
easier to mix them into the libs, as that contains some paths, too, both
in projects and in prl files.

this brings the windows generators in line with the unix ones.

Change-Id: I1f58f7edd8e21d28bfabf04384bac2e315aaf446
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>