build: Discover libepoll-shim on FreeBSD

FreeBSD doesn't offer epoll, but an implementation which uses kqueue is
available as an external library. On FreeBSD in subsystems which require
epoll have cmake look for libepoll-shim.

Type: improvement
Change-Id: Iafd5406a9e2ebaa53fd94034489ffbbf87a7d040
Signed-off-by: Tom Jones <thj@freebsd.org>