oss: FindLibEvent.cmake: workaround cmake POLICY annoyances
oss: FindLibEvent.cmake: workaround cmake POLICY annoyances

Summary:
When pulled in as an external project into an environment
that has changed the default cmake policy, the trick we're using
to extract the full library path errors out.  This works around
that issue by saving the policy settings and then setting them
how we need them before restoring them after we're done.

refs https://github.com/facebook/folly/issues/1100

Reviewed By: yfeldblum

Differential Revision: D14766991

fbshipit-source-id: e2506bc0aebd9e66bc70b12c4c538c9e034f9f73

rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp