Add back a working autogen.sh

There was no reason to get rid of the convenient script. Most developers
who are not in love with autoconf fail to remember that autoreconf
invocation, therefore it is better to have something working in place.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

autogen.sh

+#!/bin/sh
+
+autoreconf -fiv &&
+./configure