the thread library is needed only for he xotcl request monitor, background delivery or xowiki chat in a certain configuration (streaming modes, see
http://www.openacs.org/xowiki/pages/en/chat)
How comes that you say that libthread is not on sourceforge? Just tried the provided link, seems ok. Maybe there was a short problem with sourceforge again. http://sourceforge.net/project/showfiles.php?group_id=10894&package_id=10438&release_id=294044
Btw, xotcl is now at 1.4.0, please follow the install instructions in http://www.openacs.org/xowiki/pages/en/xotcl-core
The bad thing about using forum posting for install instructions is that one cannot update these, one finds as well somewhat outdated instructions.