Autoconf 2.53: improved fix for problems with CONFIG_SHELL=bash

[Aaron Ucko]

A value of "/bin/bash -norc" turned out to break certain make
implementations which didn't expect SHELL to have any options, so I
ended up settling on an alternative solution: inverting the loops in
_AS_LINENO_PREPARE and making bash a last resort, favoring sane
versions of ksh and the like.

Please disregard the suggestions I made yesterday and consider this instead.

    for as_base in sh ksh sh5 bash; do
      _AS_PATH_WALK([/bin$PATH_SEPARATOR/usr/bin$PATH_SEPARATOR$PATH],
         [case $as_dir in
         /*)
           if ("$as_dir/$as_base" -c '_AS_LINENO_WORKS') 2>/dev/null; then
             CONFIG_SHELL=$as_dir/$as_base
             export CONFIG_SHELL
             exec "$CONFIG_SHELL" "$[0]" ${1+"address@hidden"}
           fi;;
         esac
       done]);;