configure: added the copyright notice at the begining of the file

This include using the AC_COPYRIGHT macro so our copyright will also be
placed in the generated configure script.

Took opportunity to update the comment on how the file should be processed
to simplify user's life.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

configure.ac

@@ -1,13 +1,27 @@
+dnl ============================================================================
 dnl
-dnl Window Maker autoconf input.
+dnl Window Maker autoconf input
 dnl
-dnl Process with:
+AC_COPYRIGHT([Copyright (c) 2001-2015 The Window Maker Team])
-dnl               aclocal
-dnl               autoheader
-dnl               autoconf
-dnl               libtoolize --force --automake
-dnl               automake -a --gnu --include-deps
 dnl
+dnl ============================================================================
+dnl
+dnl This program is free software; you can redistribute it and/or modify
+dnl it under the terms of the GNU General Public License as published by
+dnl the Free Software Foundation; either version 2 of the License, or
+dnl (at your option) any later version.
+dnl
+dnl This program is distributed in the hope that it will be useful,
+dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
+dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+dnl GNU General Public License for more details.
+dnl
+dnl You should have received a copy of the GNU General Public License along
+dnl with this program; see the file COPYING.
+dnl
+dnl ============================================================================
+dnl
+dnl Process with: ./autogen.sh
 
 
 dnl Due to a bug in Autoconf 2.68 (apparently a regression), we need at least