Remove system specific directories from qconfig.pri

Make sure that include & library directories that are specific
to the build machine are stripped from qconfig.pri. Otherwise
they will end up in e.g. the moc calls, and might result in
fatal errors if only some of the include paths are found.

We might consider actually updating the paths at installation
time in the future, see QTBUG-31986.

Task-number: QTBUG-31496
Change-Id: I61148d381b452d3aded31eb398e590684c52f07b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>