threads madness

["Howard Chu" <hyc@symas.com>]

Solaris provides sched_yield in a separate library librt, it also has
thr_yield in libc. Currently it looks like configure only tests for thr_yield
if sched_yield doesn't exist. I'd like to turn this test around so that
thr_yield is the preferred symbol. Dragging in librt causes many undesirable
side-effects, particularly in the context of nss_ldap/pam_ldap being
dynamically loaded into a process' context.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support