autotools: try improving thread detection.

autotools: try improving thread detection.

As already stated before reliably detecting CC, LIBS and CFLAGS parameters for
use with threads is still a pain in 2014. With this patch I am trying to solve
T880 and use ax_pthread.m4. Note that I do not trust this macro more than the
previous one, so if there is any build issue on any system regarding thread, I
may just rollback to the previous code. Still I hope it is an improvement and
that our build system detect thread more reliably.