adding shared install paths to QMAKE_{INCDIR,LIBDIR} in the spec has the
tiny side effect that they are searched _first_, which is generally a
really bad idea - they should be _last_.
for that purpose, make QMAKE_{INCDIR,LIBDIR}_POST live up to their names
(i.e., search them actually last) and migrate all affected specs to use
them.
Task-number: QTBUG-40825
Change-Id: Ie0de81c3cc49e193186d2fedd7d6c77590c8ef79
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
6dcd944dee