(autoconf.info)Prerequisite Macros

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7.4.1 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])