Update the ac_define_dir macro from the autoconf archive

This macro was removed many years ago from the archive because it crosses
the boundary between configure and make time variables. For the moment
update it to the latest released version.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>