Small supporting files, short manuals (under 300 lines long) and rough documentation (README files, INSTALL files, etc.) can use a simple all-permissive license like this one:
Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.
Older versions of this license did not have the second sentence with the express warranty disclaimer. There is no urgent need to update existing files, but new files should use the new text.
If your package distributes Autoconf macros that are intended to be used (hence distributed) by third-party packages under possibly incompatible licenses, you may also use the above all-permissive license for these macros.
These kinds of files can also be put in the public domain. If publishing in the US, it is enough to insert a notice saying so. Otherwise, use Creative Commons’s CC0—See https://creativecommons.org/public-domain/cc0/. But don’t use CC0 for software code; it is not designed for such material.
If you use CC0, please include a copy of its “legal text” in a file named COPYING.CC0. See https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt. Unfortunately, that file doesn’t include licensing terms for the text of CC0, so please add this at the top of that file:
According to Creative Commons (https://creativecommons.org/publicdomain/zero/1.0/legalcode.en): The text of the Creative Commons public licenses is dedicated to the public domain under the CC0 Public Domain Dedication.