Dear Mike,
You can find useful information on libssh's failure to build below
http://permalink.gmane.org/gmane.comp.audio.csound.devel/27287
It seems that libssh's 0.6.3 pthread.c was written against an aging pthread header, so I've attached a patch.
You can also try solving the problem the other way around (which is not recommended) just to see what happens. I have included a patch for mingw32 that restores the obsolete structure. If you are brave enough to test it, remember not to apply (to un-patch) the first patch. The two patches are mutually exclusive.
-George