Time to backpatch config/ax_pthread.m4 changes?

I noticed that buildfarm member quokka (RHEL 7 / ppc64) has started
failing configure in the 9.4 and 9.5 branches. This appears to be
because configure is not detecting the need to use "-pthread" to get
thread-related functions. We didn't change anything in that area
ourselves, so I'm guessing that this is a result of a platform update,
which more than likely is going to start affecting other users before
long, so we'd better fix it.

It looks to me like the reason it's okay in 9.6 and up is
commit e97af6c8b ("Replace our hacked version of ax_pthread.m4 with
latest upstream version") plus a few followup fixes. I propose to
back-patch those into 9.5 and 9.4. Heikki had attempted to back-patch
into 9.5 originally, but gave up when the followup fixes seemed to
be getting out of hand --- but there weren't really that many.
Anyway, now that that code has been stable for several years, I see
little reason not to back-patch it.

regards, tom lane