doc: Correct statement about license of m4/*.m4 files

[Bruno Haible]

Gnulib has a number of modules
  - whose only source files are *.m4 files,
  - with no dependencies.

According to the documentation
<https://www.gnu.org/software/gnulib/manual/html_node/Copyright.html>,
all *.m4 files are under a "unlimited permission to copy and/or distribute it"
license. But this is not true: The files
  mprotect.m4
  pkg.m4
  sigaltstack.m4
  stack-direction.m4
are under the "GPL with Autoconf exception" license. There are no practical
consequences; however, the doc should be correct anyway.


2024-08-30  Bruno Haible  <bruno@clisp.org>

        doc: Correct statement about license of m4/*.m4 files.
        * doc/gnulib-intro.texi (Copyright): For m4/*.m4 files, list also the
        "GPL with Autoconf exception" as a possible license notice.

diff --git a/doc/gnulib-intro.texi b/doc/gnulib-intro.texi
index 7000fe199b..2ca732fac9 100644
--- a/doc/gnulib-intro.texi
+++ b/doc/gnulib-intro.texi
@@ -510,6 +510,21 @@
 with or without modifications, as long as this notice is preserved.
 @end quotation
 
+or under this copyright:
+
+@quotation
+Copyright @copyright{} 20XX--20YY Free Software Foundation, Inc.@*
+This file is free software, distributed under the terms of the GNU
+General Public License as published by the Free Software Foundation;
+either version 2 of the License, or (at your option) any later version.
+As a special exception to the GNU General Public License, this file
+may be distributed as part of a program that contains a configuration
+script generated by Autoconf, under the same distribution terms as
+the rest of that program.
+@end quotation
+
+We denote either of these license statements as ``unlimited''.
+
 @item tests/
 If a license statement is not present in a test module, the test files are
 under GPL@.  Even if the corresponding source module is under LGPL, this is