21.4 Features of M4

M4 includes a number of pre-defined macros that make it a powerful preprocessor. We will take a tour of the most important features provided by these macros. Although some of these features are not very relevant to GNU Autotools users, Autoconf is implemented using most of them. For this reason, it is useful to understand the features to better understand Autoconf's behavior and for debugging your own `configure' scripts.