[slapi-nis/f16] - when building for 389-ds, use Slapi_RWLocks if they appear to be available (the rest of #730394/#7

[Nalin Dahyabhai]

Summary of changes:

  986b9f9... - when building for 389-ds, use Slapi_RWLocks if they appea (*)

(*) This commit already existed in another branch; no separate mail sent