make default_pro.prf advertize dynamically created .qmake.cache

make default_pro.prf advertize dynamically created .qmake.cache

moving the detection of .qmake.cache to the qmake startup had the side
effect that a suddenly popping up cache would not be picked up by
nested projects any more.
this is not supposed to work in the first place, but the syncqt hack for
building against non-installed modules relies on it. until we have
cleaned that up properly, we need a way to notify qmake about the
appearance of the cache file.

Change-Id: I450646b936e3bb2ef2ed3aba05df58e521ccdc61
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>