add support for a super cache

qmake will now look for .qmake.super, just like it looks for .qmake.cache,
and the cache() function has a mode to write this super cache.
this allows the creation of aggregator projects like, say, qt5.
a notable difference to the normal cache is that this file is *not* added
as a dependency of the Makefile. this means that modifications done by
later sub-projects will not cause a re-processing of earlier projects, and
consequently that one should be cautious regarding what information to
store there.
another notable difference is that this file is read *before* the spec,
so the spec can use the variables from the cache without resorting to
$$fromfile() & co.

Change-Id: I4807b6d34014261fa9eebd6f0ae128b802d86691
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>

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