autoconf: Versioning

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1 4.2 Dealing with Autoconf versions
1 ==================================
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1 The following optional macros can be used to help choose the minimum
1 version of Autoconf that can successfully compile a given
1 `configure.ac'.
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1  -- Macro: AC_PREREQ (VERSION)
1      Ensure that a recent enough version of Autoconf is being used.  If
1      the version of Autoconf being used to create `configure' is
1      earlier than VERSION, print an error message to the standard error
1      output and exit with failure (exit status is 63).  For example:
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1           AC_PREREQ([2.69])
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1      This macro may be used before `AC_INIT'.
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1  -- Macro: AC_AUTOCONF_VERSION
1      This macro was introduced in Autoconf 2.62.  It identifies the
1      version of Autoconf that is currently parsing the input file, in a
11      format suitable for `m4_version_compare' (⇒
      m4_version_compare); in other words, for this release of
1      Autoconf, its value is `2.69'.  One potential use of this macro is
1      for writing conditional fallbacks based on when a feature was
1      added to Autoconf, rather than using `AC_PREREQ' to require the
1      newer version of Autoconf.  However, remember that the Autoconf
1      philosophy favors feature checks over version checks.
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1      You should not expand this macro directly; use
1      `m4_defn([AC_AUTOCONF_VERSION])' instead.  This is because some
1      users might have a beta version of Autoconf installed, with
1      arbitrary letters included in its version string.  This means it
1      is possible for the version string to contain the name of a
1      defined macro, such that expanding `AC_AUTOCONF_VERSION' would
1      trigger the expansion of that macro during rescanning, and change
1      the version string to be different than what you intended to check.
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