all: re-introduce checks for libepoll-shim

These got erroneously removed in commit 3be82964f866
A follow-up commit will adjust the nearby comments.

Reported by: jbeich
Fixes: 3be82964f866 ("Mk/bsd.port.mk: mark FreeBSD 13.4 as unsupported.")

(cherry picked from commit a3fc04a50fe513257910f8580c552d7a08277bda)