Re: zsh and autoconf-2.50

[Akim Demaille]

| > So yes, define and undefine are undefined (in M4sh and M4sugar), but
| > no, you should not know about this (in Autoconf), you should not be
| > aware of it.
| > 
| > What happens?
| 
| % autoconf2.50
| configure.in:55: error: undefine: undefined: zsh-debug
| configure.in:55: the top level
| 
| The offending construct is here.
| 
| undefine([zsh-debug])dnl
| AC_ARG_ENABLE(zsh-debug,
| [  --enable-zsh-debug         compile with debug code and debugger symbols],
| [if test x$enableval = xyes; then
|   AC_DEFINE(DEBUG)
| fi])

This 

| configure.in:55: error: undefine: undefined: zsh-debug

reads `there is an error declared by undefine: zsh-debug is
undefined'.

I agree the original `undefine' from M4 does not do that.  I agree the
message should be

| configure.in:55: error: undefine: undefined macro: zsh-debug

But by experience I know we need to be stricter to catch asap bad
bugs, so I definitely prefer the 2.50 behavior.  I'd use
ifdef(,,undefine()).