[config patch] Fwd from binutils: add ax_pthread.m4 to config/ directory

[Cary Coutant]

Please see this patch posted to the binutils list:

   https://sourceware.org/ml/binutils/2018-02/msg00260.html

where Joshua proposes to add the ax_pthread.m4 script, from the
autoconf macro archive, to the config/ directory in order to improve
gold's configure-time detection of thread support.

Is the config/ part of this patch OK?

config/
        * ax_pthread.m4: New file.

-cary