The GNU Autoconf Archive is a collection of many macros for `GNU
Autoconf that have been contributed as free software by friendly
supporters of the cause from all over the Internet.  Every single
one of those macros can be re-used without imposing any restrictions
whatsoever on the licensing of the generated `configure` script.
In particular, it is possible to use all those macros in `configure`
scripts that are meant for non-free software.