[CMAKE] Use new default for CMP0071

Summary:
Since cmake 3.18.1 cmake warns when the old policy is set:
https://cmake.org/cmake/help/latest/policy/CMP0071.html

Using the new policy does not change the build, and has no measurable
impact on performance.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7210