19.2 Why Require GNU M4?
Why does Autoconf require GNU M4?
Many M4 implementations have hard-coded limitations on the size and
number of macros that Autoconf exceeds. They also lack several
builtin macros that it would be difficult to get along without in a
sophisticated application like Autoconf, including:
Autoconf requires version 1.4.4 or later of GNU M4.
Since only software maintainers need to use Autoconf, and since GNU
M4 is simple to configure and install, it seems reasonable to require
GNU M4 to be installed also. Many maintainers of GNU and
other free software already have most of the GNU utilities
installed, since they prefer them.