m4: sync ax_pthread.m4 with autoconf archive

m4: sync ax_pthread.m4 with autoconf archive

Sync with [1].

In particular, this lets us configure with -Wunused-but-set-parameter.

[1] https://raw.githubusercontent.com/autoconf-archive/autoconf-archive/62e8491dbc174113a04fe910cad1e92e8a9e2164/m4/ax_pthread.m4

Change-Id: I496e3d9a8e896b04bf3cc87ddd41b1b144d23ee5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2128
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>