[CMAKE] Improve FindEvent

Summary:
This diff uses the facilities introduced in D5339.

It allows for checking the minimum version on a best effort basis: it
requires to run a libevent function so is not practical when cross
compiling.
It also separates the targets for libevent and libevent_pthreads,
and improves the messages.

Depends on D5339.

Test Plan:

cmake -GNinja ..

Check the output prints both the event and event_pthread library
paths, as well as the include path.

ninja all check

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5393