autoconf: Canonicalizing

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1 14.2 Getting the Canonical System Type
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1 The following macros make the system type available to `configure'
1 scripts.
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1    The variables `build_alias', `host_alias', and `target_alias' are
1 always exactly the arguments of `--build', `--host', and `--target'; in
1 particular, they are left empty if the user did not use them, even if
1 the corresponding `AC_CANONICAL' macro was run.  Any configure script
1 may use these variables anywhere.  These are the variables that should
1 be used when in interaction with the user.
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1    If you need to recognize some special environments based on their
1 system type, run the following macros to get canonical system names.
1 These variables are not set before the macro call.
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1    If you use these macros, you must distribute `config.guess' and
1 `config.sub' along with your source code.  ⇒Output, for
1 information about the `AC_CONFIG_AUX_DIR' macro which you can use to
1 control in which directory `configure' looks for those scripts.
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1  -- Macro: AC_CANONICAL_BUILD
1      Compute the canonical build-system type variable, `build', and its
1      three individual parts `build_cpu', `build_vendor', and `build_os'.
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1      If `--build' was specified, then `build' is the canonicalization
1      of `build_alias' by `config.sub', otherwise it is determined by
1      the shell script `config.guess'.
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1  -- Macro: AC_CANONICAL_HOST
1      Compute the canonical host-system type variable, `host', and its
1      three individual parts `host_cpu', `host_vendor', and `host_os'.
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1      If `--host' was specified, then `host' is the canonicalization of
1      `host_alias' by `config.sub', otherwise it defaults to `build'.
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1  -- Macro: AC_CANONICAL_TARGET
1      Compute the canonical target-system type variable, `target', and
1      its three individual parts `target_cpu', `target_vendor', and
1      `target_os'.
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1      If `--target' was specified, then `target' is the canonicalization
1      of `target_alias' by `config.sub', otherwise it defaults to `host'.
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1    Note that there can be artifacts due to the backward compatibility
1 code.  ⇒Hosts and Cross-Compilation, for more.
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