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>
diff --git a/configure.in b/configure.ac
similarity index 100%
rename from configure.in
rename to configure.ac