4.1. AC_DEFINE

The macro AC_DEFINE can be used anywhere in the script, to define a C preprocessor macro.
It accepts an optional argument indicating the value of the macro.
If you want to do shell expansion for the value, use AC_DEFINE_UNQUOTED instead.
Some Examples:

AC_DEFINE(HELLO)

AC_DEFINE(VERSION, "5.6")

myvar="foo"
othervar="bar"
AC_DEFINE_UNQUOTED(TEST, "${myvar}${othervar}")