if generating obj@ links, check if the existing obj@ link points to the

if generating obj@ links, check if the existing obj@ link points to the
same path we want to make it point to later.  If it does, leave the link
as-is and do not attempt to recreate it.  read-only /usr/src with obj@
links will work much better with this.  Fix from Niklas -- anyone who
figures out a way to do this without using perl gets a cookie.