Rename configure.in -> configure.ac

Rename configure.in -> configure.ac to accomodate newer GNU Autotools.
(http://lists.gnu.org/archive/html/autotools-announce/2012-11/msg00000.html
announced the intent to drop support for "configure.in" in future
versions of Autoconf).

Signed-off-by: Roland Dreier <roland@purestorage.com>