Fix problem with out-of-date dependencies

Fix problem with out-of-date dependencies

Problem reported by Robert Marshall in:
http://lists.gnu.org/archive/html/emacs-devel/2017-03/msg00501.html
Although this problem has been with us for a while, the recent
change from Automake to GNU Make exposed it again.
* configure.ac (AUTO_DEPEND): When autodepending, clean out any
leftover dependency and object files, since the previous sources'
dependencies may disagree with the current ones.  Reconfiguring
typically needs to force a rebuild anyway.