Remove autoconf-specific files from .gitignore

Change-Id: I4495967653e99ff50c7b75f2290d6bb44397fc08
Reviewed-on: http://review.couchbase.org/33981
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>