move setting of _QMAKE_{SUPER,CONF,CACHE}_ to a more logical location

there is no point in setting the variables already when peeking into
the caches, as that is done in a separate evaluator anyway.
it also makes no sense to have them set while loading the spec itself,
as it's not permitted to do anything with the caches.
so set them at the next convenient point, which is right before actually
loading the caches.

Change-Id: I3717ddf94353dc04e12c52e542f16ed27b578e14
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/0f68222e9ebdf42778d4bb84260268764eb55b30)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>