set QTDIR_build in .qmake.super

this suppresses license checking in default_pre.prf in the super module.
submodules set it via their .qmake.conf's.

Change-Id: Ic3599613140169eeacb48385be8f324c4bef8f1a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qt.pro

 # Create the super cache so modules will add themselves to it.
-cache(, super)
+# Also suppress the license check on subsequent "visits". The first
+# visit will skip it anyway due to not having a compiler set up yet.
+cache(CONFIG, add super, $$list(QTDIR_build))
 
 TEMPLATE      = subdirs