[m-rev.] diff: do not warn about obsolete autoconf macros

[Julien Fischer]

>From autoconf 2.70 onwards, obsolescence warnings are enabled by default.
Recently I have fixed all the warnings we were getting, except for:

1. With autoconf 2.70 we get a warning about our use of AC_PROG_LEX;
this cannot be avoided while preserving support for autoconf 2.69

2. Warnings about escaping of quotes and double quotes, like:

     warning: back quotes and double quotes must not be escaped in:
     $as_me:${as_lineno-$LINENO}: WARNING: cannot find a working \`mkfifo' or \`mknod'

I haven't (yet) found a way to avoid this warning and get a configure
script that works :-(  Suggestions welcome.

-------------

Do not warn about obsolete autoconf macros.

prepare.sh:
     As above.

Julien.

diff --git a/prepare.sh b/prepare.sh
index 8e454cd..2ca5c05 100755
--- a/prepare.sh
+++ b/prepare.sh
@@ -16,4 +16,4 @@ fi

  echo "Setting up autotools"
  aclocal -I m4
-autoconf
+autoconf -Wno-obsolete