Bump to C++20 standard (with GNU extensions)

In practice, for the time being, we limit ourselves to C++20 features available
in GCC 10 and clang 12 (the latter because of the Julia infrastructure).

configure.ac

 dnl Process this file with autoconf to produce a configure script.
 
-dnl Copyright © 2009-2021 Dynare Team
+dnl Copyright © 2009-2022 Dynare Team
 dnl
 dnl This file is part of Dynare.
 dnl
@@ -25,7 +25,6 @@ AM_INIT_AUTOMAKE([1.11 -Wall -Wno-portability foreign no-dist-gzip dist-xz tar-p
 
 AC_PROG_CC
 AC_PROG_CXX
-AX_CXX_COMPILE_STDCXX_17
 
 AC_CANONICAL_HOST
 case ${host_os} in
@@ -44,7 +43,7 @@ esac
 # Use C++ for testing headers
 AC_LANG([C++])
 
-AM_CXXFLAGS="-Wall -Wno-parentheses -Wold-style-cast"
+AM_CXXFLAGS="-std=gnu++20 -Wall -Wno-parentheses -Wold-style-cast"
 AC_SUBST([AM_CXXFLAGS])
 
 # If default 'ar' is not available, try to find one with a host prefix (see ticket #145)

m4/ax_cxx_compile_stdcxx_17.m4

-# =============================================================================
-#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_17.html
-# =============================================================================
-#
-# SYNOPSIS
-#
-#   AX_CXX_COMPILE_STDCXX_17([ext|noext], [mandatory|optional])
-#
-# DESCRIPTION
-#
-#   Check for baseline language coverage in the compiler for the C++17
-#   standard; if necessary, add switches to CXX and CXXCPP to enable
-#   support.
-#
-#   This macro is a convenience alias for calling the AX_CXX_COMPILE_STDCXX
-#   macro with the version set to C++17.  The two optional arguments are
-#   forwarded literally as the second and third argument respectively.
-#   Please see the documentation for the AX_CXX_COMPILE_STDCXX macro for
-#   more information.  If you want to use this macro, you also need to
-#   download the ax_cxx_compile_stdcxx.m4 file.
-#
-# LICENSE
-#
-#   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
-#   Copyright (c) 2016 Krzesimir Nowak <qdlacz@gmail.com>
-#
-#   Copying and distribution of this file, with or without modification, are
-#   permitted in any medium without royalty provided the copyright notice
-#   and this notice are preserved. This file is offered as-is, without any
-#   warranty.
-
-#serial 2
-
-AX_REQUIRE_DEFINED([AX_CXX_COMPILE_STDCXX])
-AC_DEFUN([AX_CXX_COMPILE_STDCXX_17], [AX_CXX_COMPILE_STDCXX([17], [$1], [$2])])