[xml] LIBXML_THREAD_ENABLED and HAVE_PTHREAD_H mismatch

[Kwasi Mensah <kwasi mensah gmail com>]

I ran into the issue where HAVE_PTHREAD_H is defined but LIBXML_THREAD_ENABLED wasn't. The fix was already submitted as per http://mail.gnome.org/archives/xml/2005-August/msg00136.html .

Out of curiosity, is there a reason this patch didn't get into that main line?

This is with libxml2 2.7.8 on Mac OS X. I ran the configure script but I'm not using the make scripts as I have to make a static library that will work on the iPhone. 

Thanks,
kwasi