GNU `m4` is an implementation of the traditional Unix macro processor. It is mostly SVR4 compatible, although it has some extensions (for example, handling more than 9 positional parameters to macros). `m4` also has built-in functions for including files, running shell commands, doing arithmetic etc. Autoconf needs GNU `m4` for generating `configure` scripts, but not for running them.

WWW: https://www.gnu.org/software/m4
