.gitignore updates; rename configure.in->.ac

Add some entries to config/.gitignore for newer versions of the GNU
Autotools.

Also 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: Jeff Squyres <jsquyres@cisco.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
2 files changed