configure

This commit back-patches these 9.6-era commits into 9.4 and 9.5:

e97af6c8 Replace our hacked version of ax_pthread.m4 with latest upstream version.
3b14a17c Move pthread-tests earlier in the autoconf script.
01051a98 Use AS_IF rather than plain shell "if" in pthread-check.
a2932283 Update ax_pthread.m4 to an experimental draft version from upstream.

The net result is to sync configure's checks for threading-related
flags and libraries with the version we've been using since 9.6.
The motivation for doing so now is that it seems the older code does
not work correctly on very recent RHEL7/ppc64, as evidenced by
buildfarm member quokka.  The newer code is pretty battle-hardened
by now, so this seems like a low-risk fix.

Discussion: https://postgr.es/m/3320.1542647565@sss.pgh.pa.us