find .qmake.cache for subprojects independently

find .qmake.cache for subprojects independently

this allows the creation of aggregator projects, like, say, qt5.
this is not expected to have a negative impact, as no project could
reasonably expect a nested .qmake.cache to *not* take effect - in fact,
if the project was processed stand-alone, it would already use it.

Change-Id: I33f2935d309baba7e95465f2fefb8231c4f03eda
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>