merge revision(s) 4250d7c9,83f96642,447feb0a,1d80d703,e271a3d4,366e88e5,d825e349,4250d7c9,f9e43782: [Backport #17723]

	autogen.sh: Added

	Currently it just runs "autoreconf --install"
	---
	 aclocal.m4 | 4 ++--
	 autogen.sh | 3 +++
	 2 files changed, 5 insertions(+), 2 deletions(-)
	 create mode 100755 autogen.sh

	autogen.sh: Allow AUTORECONF envvar to specify the autoreconf
	 executable

	---
	 autogen.sh | 2 +-
	 1 file changed, 1 insertion(+), 1 deletion(-)

	Improved autogen.sh

	* update in the source directory
	* make symbolic links instead of copies
	* forward the arguments to autoreconf as-is
	---
	 autogen.sh | 8 +++++++-
	 1 file changed, 7 insertions(+), 1 deletion(-)

	autogen.sh: Make it work with sh (not bash)

	---
	 autogen.sh | 2 +-
	 1 file changed, 1 insertion(+), 1 deletion(-)

	[DOC] Update README files to use autoreconf [ci skip]

	---
	 README.ja.md | 3 +--
	 README.md    | 3 +--
	 2 files changed, 2 insertions(+), 4 deletions(-)

	[DOC] Update README [ci skip]

	* Use `autogen.sh`.
	* The subversion repository is only for old versions, and
	  secondary now.
	* Moved long links to footnotes.
	---
	 README.ja.md | 22 +++++++++-------------
	 README.md    | 11 +++++++----
	 2 files changed, 16 insertions(+), 17 deletions(-)

	Use autogen.sh

	---
	 .github/workflows/baseruby.yml           | 2 +-
	 .github/workflows/check_dependencies.yml | 2 +-
	 .github/workflows/compilers.yml          | 2 +-
	 .github/workflows/macos.yml              | 2 +-
	 .github/workflows/mjit.yml               | 2 +-
	 .github/workflows/ubuntu.yml             | 2 +-
	 6 files changed, 6 insertions(+), 6 deletions(-)

	autogen.sh: Added

	Currently it just runs "autoreconf --install"
	---
	 aclocal.m4 | 4 ++--
	 autogen.sh | 3 +++
	 2 files changed, 5 insertions(+), 2 deletions(-)
	 create mode 100755 autogen.sh

	get rid of aclocal (#4280)

	---
	 aclocal.m4   | 47 -----------------------------------------------
	 configure.ac | 35 ++++++++++++++++++++++++++++++++++-
	 2 files changed, 34 insertions(+), 48 deletions(-)