print warning for unrecognized --enable and --with options

[Steven G. Johnson]

As discussed on the autoconf mailing list, the current behavior of 
silently ignoring unrecognized --enable and --with options is rather 
unfriendly and unexpected, and is a frequent source of user error.  The 
following patch corrects this flaw, without violating the GNU Coding 
Standards or breaking recursive configuration.

Specifically, the patch causes configure to emit a warning when 
unrecognized --with and --enable options are encountered.  This warning 
can be disabled in three ways.  First, if the user passes 
--disable-option-checking.  Second, if configure.ac calls 
AC_DISABLE_OPTION_CHECKING.  Third, if configure.ac calls 
AC_CONFIG_SUBDIRS (which also passes --disable-option-checking to 
recursive configures).

Conversely, if the user passes --enable-option-checking=fatal, then 
configure halts with an error when unrecognized options are encountered.
--enable-option checking can also be used to turn the warnings back on 
even if they have been disabled by configure.ac as above.

When a warning is printed, it is printed twice: once at the beginning of 
the configure output (as soon as options are parsed), and once at the 
end.  Rationale: it is too easy to miss warnings at the beginning of 
configure output, and unrecognized options are almost always errors 
(except for recursive configures).

ChangeLog entry follows, and cvs diff -u against CVS HEAD is attached.

Cordially,
Steven G. Johnson

2006-12-16  Steven G. Johnson  <address@hidden>

        * lib/autoconf/general.m4 (_AC_INIT_DEFAULTS, _AC_INIT_PARSE_ARGS),
          _AC_INIT_PARSE_ENABLE2, _AC_INIT_HELP, _AC_ENABLE_IF):
          Print warning for unrecognized --with and --enable options
          (AC_DISABLE_OPTION_CHECKING): New macro to disable warnings.
        * lib/autoconf/status.m4 (AC_CONFIG_SUBDIRS, _AC_OUTPUT_SUBDIRS):
          Disable option checking when subdirs are configured.
          (AC_OUTPUT): If warnings are enabled, print warning about
          unrecognized --with and --enable options at the end of
          the configure output (as well as at the beginning).
        * doc/autoconf.texi: Document new option warning functionality.

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