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 builtin functions for including files, running
 shell commands, doing arithmetic, etc. Autoconf needs GNU `m4' for
 generating `configure' scripts, but not for running them.