build: Drop deprecated ACLOCAL_AMFLAGS variable #18333

This PR is a followup of #18290.

The special make variable ACLOCAL_AMFLAGS is deprecated since Automake 1.13 release. AC_CONFIG_MACRO_DIR macro is traced by aclocal, and is used to declare the local m4 include directories.

For the reference:

• GNU Automake 1.13 released
• GNU Automake 1.14 released

Concept ACK, thanks for keeping track of these deprecations!

Here's the relevant excerpt from the automake 1.14 release notes:

  - The ACLOCAL_AMFLAGS special make variable will be fully deprecated in
    Automake 2.0: it will raise warnings in the "obsolete" category (but
    still no hard error of course, for compatibilities with the many, many
    packages that still rely on that variable).  You are advised to
    start relying on the new Automake support for AC_CONFIG_MACRO_DIRS
    instead (which was introduced in Automake 1.13).

It seems that we're already using AC_CONFIG_MACRO_DIR{,S} here https://github.com/bitcoin/bitcoin/blob/ef8cdb0e6cb2c019a2e4a05aa15cc463a10477d7/configure.ac#L15

~0. I realize I pointed this out in the previous PR. However keeping this as is is harmless, and dropping it just introduces a warning:

glibtoolize: Consider adding '-I build-aux/m4' to ACLOCAL_AMFLAGS in Makefile.am.

that someone will just as likely try and fix.

From libtool docs:

If libtoolize detects an explicit call to AC_CONFIG_MACRO_DIRS ... in your configure.ac, it will put the Libtool macros in the specified directory.

Unless --no-warn is passed, libtoolize displays hints for adding libtool support to your package, as well.

I think that is a hint, not a warning. @fanquake

... someone will just as likely try and fix.

Yeah, the maintainers' burden could be increased. So, it is up to you to close this PR.

Let's come back to this in the future.