CONFIG_SHELL behavior

[Ralf Wildenhues]

CONFIG_SHELL is IMVHO used (or documented) somewhat inconsistently
by Automake.  If the user specifies it on the command line, e.g.
  CONFIG_SHELL=/bin/foosh ./configure

then it is used (among other uses) for subsequent re-checking, but
the first configure invocation is possibly not done with foosh.
This would require the user to issue
  CONFIG_SHELL=/bin/foosh /bin/foosh ./configure

or the m4sh setup to do some (possibly unnecessary) re-execution.

Now I know current Autoconf docs do not specify above use, but users
imply it, and Libtool would benefit from a solution to this.
There, one problem is that $ECHO is computed for the shell run at
configure time, but that value might not be valid for $CONFIG_SHELL.

What do you think is more appropriate: another exec or a doc update?
A solution for Libtool only is possible as well, but not as cleanly
(it would imply more overhead).

Regards,
Ralf