There is no need for this variable to be stored in CMake's cache. We
don't perform expensive operations to set up QT_SYNCQT, and we even
unset the cache variable to ensure it gets recomputed on
reconfiguration.
We still store QT_SYNCQT in a global property, because the function
qt_ensure_sync_qt is called in different directory scopes, and we want
to avoid re-calculations for every subdir.
It's now possible for the user to set QT_SYNCQT (see QTBUG-88088 for
motivation). Also, in a non-prefix build, changes to syncqt.pl in the
source dir are reflected upon re-configuration in the build
tree (because qt_copy_or_install is called on every configure).
Pick-to: 6.2
Fixes: QTBUG-88088
Task-number: QTBUG-75290
Change-Id: I6137b060d200d3dafd4a64d5a6c1bd2549723d78
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
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