configure.ac: rename configure.in to .ac Change deprecated .in extension to .ac to silent warning. https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Writing-Autoconf-Input.html Change-Id: Ic6ef8374ddc927481262c982a2579cd45dc41b84 Signed-off-by: Gaƫtan Harter <gaetan.harter@inria.fr>