autoconf-archive: backport patch for C++23 support

Backport upstream patch for C++23 support on ax_cxx_compile_stdcxx.m4.

This is needed by Asterisk package to correctly configure.

It's currently not possible to bump to a more recent autoconf-archive
release 2024.10.16 as it's currently affected by multiple BUGs ([1])

[1] openwrt/openwrt@e1acc7a

Signed-off-by: Christian Marangi <[email protected]>

Author: Ansuel

tools/autoconf-archive/patches/001-AX_CXX_COMPILE_STDCXX-Add-C-23-support.patch

@@ -0,0 +1,90 @@
+From 833c9c2e5b65383c8c1353b6beb88f52dede640f Mon Sep 17 00:00:00 2001
+From: Olly Betts <[email protected]>
+Date: Wed, 29 May 2024 07:54:24 +1200
+Subject: [PATCH] AX_CXX_COMPILE_STDCXX: Add C++23 support
+
+---
+ m4/ax_cxx_compile_stdcxx.m4 | 47 ++++++++++++++++++++++++++++++++++---
+ 1 file changed, 44 insertions(+), 3 deletions(-)
+
+--- a/m4/ax_cxx_compile_stdcxx.m4
++++ b/m4/ax_cxx_compile_stdcxx.m4
+@@ -10,8 +10,8 @@
+ #
+ #   Check for baseline language coverage in the compiler for the specified
+ #   version of the C++ standard.  If necessary, add switches to CXX and
+-#   CXXCPP to enable support.  VERSION may be '11', '14', '17', or '20' for
+-#   the respective C++ standard version.
++#   CXXCPP to enable support.  VERSION may be '11', '14', '17', '20', or
++#   '23' for the respective C++ standard version.
+ #
+ #   The second argument, if specified, indicates whether you insist on an
+ #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
+@@ -43,7 +43,7 @@
+ #   and this notice are preserved.  This file is offered as-is, without any
+ #   warranty.
+ 
+-#serial 18
++#serial 19
+ 
+ dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
+ dnl  (serial version number 13).
+@@ -53,6 +53,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
+         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
+         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
+         [$1], [20], [ax_cxx_compile_alternatives="20"],
++        [$1], [23], [ax_cxx_compile_alternatives="23"],
+         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
+   m4_if([$2], [], [],
+         [$2], [ext], [],
+@@ -186,6 +187,16 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbo
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
+ )
+ 
++dnl  Test body for checking C++23 support
++
++m4_define([_AX_CXX_COMPILE_STDCXX_testbody_23],
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_23
++)
++
+ 
+ dnl  Tests for new features in C++11
+ 
+@@ -1016,3 +1027,33 @@ namespace cxx20
+ #endif  // __cplusplus < 202002L && !defined _MSC_VER
+ 
+ ]])
++
++
++dnl  Tests for new features in C++23
++
++m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_23], [[
++
++#ifndef __cplusplus
++
++#error "This is not a C++ compiler"
++
++#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
++
++#error "This is not a C++23 compiler"
++
++#else
++
++#include <version>
++
++namespace cxx23
++{
++
++// As C++23 supports feature test macros in the standard, there is no
++// immediate need to actually test for feature availability on the
++// Autoconf side.
++
++}  // namespace cxx23
++
++#endif  // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
++
++]])