autoconf: Option Checking

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1 15.5 Controlling Checking of `configure' Options
1 ================================================
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1 The `configure' script checks its command-line options against a list
1 of known options, like `--help' or `--config-cache'.  An unknown option
1 ordinarily indicates a mistake by the user and `configure' halts with
1 an error.  However, by default unknown `--with-PACKAGE' and
1 `--enable-FEATURE' options elicit only a warning, to support
1 configuring entire source trees.
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1    Source trees often contain multiple packages with a top-level
11 `configure' script that uses the `AC_CONFIG_SUBDIRS' macro (⇒
 Subdirectories).  Because the packages generally support different
1 `--with-PACKAGE' and `--enable-FEATURE' options, the GNU Coding
1 Standards say they must accept unrecognized options without halting.
1 Even a warning message is undesirable here, so `AC_CONFIG_SUBDIRS'
1 automatically disables the warnings.
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1    This default behavior may be modified in two ways.  First, the
1 installer can invoke `configure --disable-option-checking' to disable
1 these warnings, or invoke `configure --enable-option-checking=fatal'
1 options to turn them into fatal errors, respectively.  Second, the
1 maintainer can use `AC_DISABLE_OPTION_CHECKING'.
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1  -- Macro: AC_DISABLE_OPTION_CHECKING
1      By default, disable warnings related to any unrecognized
1      `--with-PACKAGE' or `--enable-FEATURE' options.  This is implied
1      by `AC_CONFIG_SUBDIRS'.
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1      The installer can override this behavior by passing
1      `--enable-option-checking' (enable warnings) or
1      `--enable-option-checking=fatal' (enable errors) to `configure'.
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