Test honestly for <sys/signalfd.h>.

Commit 6a2a70a02 supposed that any platform having <sys/epoll.h>
would also have <sys/signalfd.h>.  It turns out there are still a
few people using platforms where that's not so, so we'd better make
a separate configure probe for it.  But since it took this long to
notice, I'm content with the decision to not have a separate code
path for epoll-only machines; we'll just fall back to using poll()
for these stragglers.

Per gripe from Gabriela Serventi.  Back-patch to v14 where this
code came in.

Discussion: https://postgr.es/m/CAHOHWE-JjJDfcYuLAAEO7Jk07atFAU47z8TzHzg71gbC0aMy=g@mail.gmail.com

configure.ac

@@ -1406,6 +1406,7 @@ AC_CHECK_HEADERS(m4_normalize([
 	sys/select.h
 	sys/sem.h
 	sys/shm.h
+	sys/signalfd.h
 	sys/sockio.h
 	sys/tas.h
 	sys/uio.h