Prevent some combinations of autotools and libtool version

from generating a non-working top level 'libtool'.


ltmain.sh:
  Some combinations of autotools and libtool leave 'max_cmd'len'
  (in top-level 'libtool') unset, this eventually causes a
  'ld' command without input files to be generated.
  Prevent this error by supplying a 4 kB default value.

ltmain.sh

@@ -51,6 +51,9 @@ fi
 # libtool 1.4.2 workaround
 SED=${SED:-sed}
 
+# workaround against unset 'max_cmd_len': assume at least 4 kB
+max_cmd_len=${max_cmd_len:-4096}
+
 # The name of this program.
 progname=`$echo "$0" | ${SED} 's%^.*/%%'`
 modename="$progname"