ci: Update autoconf to version 2.73

[gsnw-sebast]

A new version of autoconf was released on Fri, Mar 20, 2026, at 3:47:42 PM -0400

https://lists.gnu.org/archive/html/autoconf/2026-03/msg00060.html

[gemini-code-assist]

Summary of Changes

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This pull request performs a routine maintenance update by bumping the Autoconf version used in the project's CI pipeline. This ensures that the build process leverages the latest features and bug fixes provided by the new Autoconf release, contributing to a more robust and up-to-date development environment.

Highlights

• Autoconf Version Update: The continuous integration (CI) configuration has been updated to utilize Autoconf version 2.73, incorporating the latest release for improved build processes.

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[gemini-code-assist]

Code Review

This pull request updates autoconf to version 2.73. The changes are straightforward, correctly updating the version in the CI build script and the changelog. I have added one suggestion to the build script to define tool versions in variables, which should improve maintainability for future dependency updates.

[gemini-code-assist]

To improve maintainability, consider defining the tool versions in separate variables. This makes it easier to see all versions at a glance and simplifies future updates.

Suggested change

	AUTOCONF_REL=autoconf/autoconf-2.73.tar.gz
	AUTOMAKE_REL=automake/automake-1.18.1.tar.gz
	LIBTOOL_REL=libtool/libtool-2.5.4.tar.gz
	AUTOCONF_VERSION=2.73
	AUTOMAKE_VERSION=1.18.1
	LIBTOOL_VERSION=2.5.4
	AUTOCONF_REL=autoconf/autoconf-${AUTOCONF_VERSION}.tar.gz
	AUTOMAKE_REL=automake/automake-${AUTOMAKE_VERSION}.tar.gz
	LIBTOOL_REL=libtool/libtool-${LIBTOOL_VERSION}.tar.gz

[coveralls]

coverage: 88.152% (+0.1%) from 88.037%
when pulling a1c7e66 on gsnw:ci-update-autotools
into b862609 on schweikert:develop.