Add helper script to clean top level dir from autoconf stuff

Change-Id: I0646976ee28f02272f9da94c5304fa56e9649d7a
Reviewed-on: http://review.couchbase.org/43805
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>