rename configure.in->ac

Newer autotools will reject the old name, so use the new one.

BUG=None
TEST=precq passes

Change-Id: I1eaad01b07e92d18e13af15ff7efdcef624205e4
Reviewed-on: https://chromium-review.googlesource.com/271630
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Darren Krahn <dkrahn@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>