we were finding features/ directly in the source & build root, and we
were finding features/ under mkspecs/ from $QMAKEPATH and other mkspecs
locations, but we omitted the "transitive hull". this was
counterintuitive.

Change-Id: I9823e6606467c98f264c81385250da92311f51ca
(cherry picked from qtbase/0bf65180b8710e4516496d0bc53a265ff0dd6a01)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>