proposal - command-line option checking

[Steven G. Johnson]

Something that has been bothering me for a long time has been Autoconf's 
lack of checking for command-line options.  That is, it doesn't give any 
error, or even a warning, if the user accidentally types --with-foo or 
--enable-ffoo instead of --enable-foo.  In my experience, this has 
caused numerous problems because users think they've enabled things when 
they haven't.  (Heck, it's burned me several times on my own programs!)

Not only is it unfriendly, it's also unexpected: virtually every other 
Unix command-line program complains if you pass it an unknown argument.

Yes, I know that programs which use AC_CONFIG_SUBDIRS need to pass 
options through without checking since they might be used in 
subdirectories.  But these are the exception, not the rule, and it 
doesn't make sense to penalize most users for this.

Hence, PROPOSAL:

   * Autoconf configure scripts should check their --with and --enable 
arguments, and exit with an error if the option was not specified in an 
AC_ARG_ENABLE or AC_ARG_WITH statement, unless:

   * This option-checking is disabled if AC_CONFIG_SUBDIRS is used. 
(At least initially; a future implementation may check the available 
options in the subdirectories, if a good way can be found to do this.)

   * The option checking can be disabled by using a new macro, 
AC_DISABLE_OPTION_CHECKING, or by passing --disable-option-checking, for 
those people who prefer or require the legacy behavior for some reason.

What do people think?  I don't want to code up a patch until there is 
some consensus about the desired functionality.

Steven G. Johnson