(autoconf.info)Macro Definitions

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7.1 Macro Definitions
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Autoconf macros are defined using the 'AC_DEFUN' macro, which is similar
to the 'm4' builtin 'define' macro.  In addition to defining a macro,
'AC_DEFUN' adds to it some code which is used to constrain the order in
which macros are called (Note: Prerequisite Macros).

   An Autoconf macro definition looks like this:

     AC_DEFUN(MACRO-NAME, [MACRO-BODY])

The square brackets here do not indicate optional text: they should
literally be present in the macro definition to avoid macro expansion
problems (Note: Quoting).  You can refer to any arguments passed to
the macro as '$1', '$2', etc.

   To introduce comments in 'm4', use the 'm4' builtin 'dnl'; it causes
'm4' to discard the text through the next newline.  It is not needed
between macro definitions in 'acsite.m4' and 'aclocal.m4', because all
output is discarded until 'AC_INIT' is called.

   Note: How to define new macros, for more
complete information on writing 'm4' macros.