find qt.conf more reliably

find qt.conf more reliably

a qmake invoked with a relative path would not find qt.conf, because it
already changed CWD, which would confuse our copy of
QCoreApplication::applicationFilePath() (see also QTBUG-21381).
we already have the absolute path to the qmake executable in
Option::qmake_abslocation, so just use it.

(cherry picked from commit c378e16e57e8ce2255116e7fdb5436524cccb8c7)

Change-Id: Icfe4254722e0855f6755b639f4c86f15bc7323ba
Reviewed-on: http://codereview.qt-project.org/4565
Reviewed-by: Harald Fernengel <harald.fernengel@nokia.com>