pgsql: Test honestly for <sys/signalfd.h>.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c5f5b4dd4b5ccecd193550530ff483f16f95746b

Modified Files
--------------
configure | 2 +-
configure.ac | 1 +
src/backend/storage/ipc/latch.c | 9 ++++-----
src/include/pg_config.h.in | 3 +++
src/tools/msvc/Solution.pm | 1 +
5 files changed, 10 insertions(+), 6 deletions(-)