Update ax_pthread.m4 to an experimental draft version from upstream.

The current version is adding a spurious -pthread option on some Darwin
systems that don't need it, which leads to a bunch of "unrecognized option
'-pthread'" warnings. There is a proposed fix for that in the upstream
autoconf archive's bug tracker, see https://savannah.gnu.org/patch/?8186.
This commit updates our version of ax_pthread.m4 to the "draft2" version
proposed there by Daniel Richard G. I'm using our buildfarm to help Daniel
to test this, before he commits this to the upstream repository.