Policy CMP0189, introduced by commit b3da9c6d60 (GenEx: Evaluate
LINK_LIBRARIES target properties transitively, 2025-02-24,
v4.1.0-rc1~731^2), takes effect at generation time, and so uses the
policy value as of the end of each directory. However, some projects
may rely on `file(GENERATE)` with the policy's OLD behavior in order
to extract targets' *direct* dependencies from `LINK_LIBRARIES`.
Pending a first-class solution to that problem, make it easier for
projects to port to the policy's NEW behavior in general while
retaining the OLD behavior in an isolated context.
Fixes: #27220