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(autoconf.info)Prerequisite Macros

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Prerequisite Macros
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   A macro that you write might need to use values that have previously
been computed by other macros.  For example, `AC_DECL_YYTEXT' examines
the output of `flex' or `lex', so it depends on `AC_PROG_LEX' having
been called first to set the shell variable `LEX'.

   Rather than forcing the user of the macros to keep track of the
dependencies between them, you can use the `AC_REQUIRE' macro to do it
automatically.  `AC_REQUIRE' can ensure that a macro is only called if
it is needed, and only called once.

 - Macro: AC_REQUIRE (MACRO-NAME)
     If the `m4' macro MACRO-NAME has not already been called, call it
     (without any arguments).  Make sure to quote MACRO-NAME with
     square brackets.  MACRO-NAME must have been defined using
     `AC_DEFUN' or else contain a call to `AC_PROVIDE' to indicate that
     it has been called.

   An alternative to using `AC_DEFUN' is to use `define' and call
`AC_PROVIDE'.  Because this technique does not prevent nested messages,
it is considered obsolete.

 - Macro: AC_PROVIDE (THIS-MACRO-NAME)
     Record the fact that THIS-MACRO-NAME has been called.
     THIS-MACRO-NAME should be the name of the macro that is calling
     `AC_PROVIDE'.  An easy way to get it is from the `m4' builtin
     variable `$0', like this:

          AC_PROVIDE([$0])