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/choose/zero/.