[CMAKE] Propagate requirements for cmake >= 3.12

Summary:
D4229 introduces the requirement for cmake >= 3.12.
Take advantages of the new features available from cmake >= 3.12 to
simplify the code a bit. Also the new project properties come in handy
when using CPack.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4257