causes ldap_pvt_thread_self to be called with the wrong prototype.

That can cause OpenSSL to use a garbage value, e.g. if the unsigned
long it expects takes two words but ldap_pvt_thread_t is an int.

I'm fixing it in HEAD now and also provoking an error if unsigned
long cannot hold a ldap_pvt_thread_t.  Otherwise it can silently
compile to broken code.  Maybe the latter should go in configure,
but since OpenSSL presumably breaks anyway if that fails I don't
see much point at this time.