[autotools] Remove second call to AM_INIT_AUTOMAKE.

Complained by autoconf 1.14, cf. FS#1462.

configure.ac

-#! /bin/bash
-# $Id$
+# -*- Autoconf -*-
 # Process this file with autoconf to produce a configure script.
+
 DUNE_AC_INIT # gets module version from dune.module file
-AM_INIT_AUTOMAKE
 AC_CONFIG_SRCDIR([dune/istl/istlexception.hh])
 AC_CONFIG_HEADERS([config.h])