Now let's turn our attention back to the contents of the configure.ac file in Example 3-11. The first section contains Autoconf initialization macros. These are required for all projects. Let's consider each of these macros individually, because they're all important.
AC_PREREQ macro simply defines the earliest version of Autoconf that may be used to successfully process this configure.ac file:
The GNU Autoconf Manual indicates that
AC_PREREQ is the only macro that may be used before
AC_INIT. This is because it's good to ensure you're using a new enough version of Autoconf before you begin processing any other macros, which may be version dependent.
AC_INIT macro, as ...