autoreconf has been available since autoconf 2.50 and we probably
depend on newer versions of autoconf already.
This doesn't solve any immediate problem, except that OS X users
can now use autogen.sh directly, as explained in e2cd3ad7 (configure.ac:
use non-deprecated form of AM_INIT_AUTOMAKE, Fri May 20 22:44:34 2011 +0200).

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>