darwin 5.5 echo probes leaving empty SHELL

[Kevin Ryde]

On darwin 5.5, with the latest libtool and autoconf 2.57, it seems
_LT_AC_PROG_ECHO_BACKSLASH can leave SHELL and CONFIG_SHELL set to an
empty string.  This makes config.status creation bomb, and of course
is a bad thing overall.

The echo testing gets up to fallback-echo using ORIGINAL_CONFIG_SHELL,
but with that variable unset.  The test succeeds because the configure
script is executable and the consequent code sets SHELL and
CONFIG_SHELL to $ORIGINAL_CONFIG_SHELL, ie. an empty string.

It seems ORIGINAL_CONFIG_SHELL was not set because the system doesn't
have a /bin/ksh, and it's only under the ksh probes that it's used.
(I know darwin used to have /bin/sh as ksh, but on the machine I've
got it looks like ash now.)

In any case I think the fix is as simple as

        * libtool.m4 (_LT_AC_PROG_ECHO_BACKSLASH): Try ORIGINAL_CONFIG_SHELL
        with --fallback-echo only if that variable is set.

libtool.m4.original-config-shell.diff
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