Introduce a new macro qt_internal_return_unless_building_tools which
simply calls return() if tools are not built. This macro is supposed to
be called after qt_internal_add_tool().
Using this macro avoids having to special-case code for when
qt_internal_add_tool() creates imported targets in cross-builds.
Adjust pro2cmake accordingly.
Task-number: QTBUG-85084
Change-Id: I9e1c455c29535dd8c318efa890ebd739c42effc1
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
a0e56294c1