make sure qmake finds itself in CWD on windows

and consequently that it finds qt.conf.

Task-number: QTBUG-30583
Change-Id: I48441477e941d9609270d6e5e1b405127c0c0aca
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/e7c3b21b7c1ed9247cb2f9eeddc154534a038549)