20.4. Features of M4
M4 includes a number of predefined macros that make it a powerful preprocessor. This section takes you on 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.
20.4.1. Discarding Input
A macro called dnl discards text from the input. The dnl macro takes no arguments and expands to the empty string, but it has the side effect of discarding all input up to and including the next newline character. Here is an example of dnl from the Autoconf source ...