[xml] Threading on by default

[Dave Beckett <dave beckett bristol ac uk>]

I noticed that in 2.5.7 " threads now default to on" in the libxml2
configure (April 22) and that caused me debugging problems when I was
using it on Debian, requiring me to build a thread-free version. (It is
the first time I had to kill -9 gdb).

I'm curious why the default was switched.

Cheers

Dave