Reduce the directories plugins are searched in

Don't search in the mac directories on linux/unix and vice versa.
Orginal patch by Ray Donnelly <mingw.android@laptop.com> on the android
branch.

Change-Id: I57a4d83a94f28bbe00609fd1b9170629da27334e
Reviewed-on: http://codereview.qt.nokia.com/3949


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>