* The autoconf-archive package contains macros useful for detecting C++
standard and testing other C++ capabilities but that package was never
included in the install_prereq script so many existing build environments
won't have it. Even if it is installed, older versions won't newer C++
standards and will actually cause an error if you try to test for that
version. To make it available for those environments, the
ax_cxx_compile_stdcxx.m4 macro has copied from the latest release of
autoconf-archive into the autoconf directory.
* A convenience wrapper(ast_cxx_check_std) around ax_cxx_compile_stdcxx was
also added so checking the standard version and setting the
asterisk-specific PBX_ variables becomes a one-liner:
`AST_CXX_CHECK_STD([std], [force_latest_std])`.
Calling that with a version of `17` for instance, will set PBX_CXX17
to 0 or 1 depending on whether the current c++ compiler supports stdc++17.
HAVE_CXX17 will also be 'defined" or not depending on the result.
* C++ compilers hardly ever default to the latest standard they support. g++
version 14 for instance supports up to C++23 but only uses C++17 by default.
If you want to use C++23, you have to add `-std=gnu++=23` to the g++
command line. If you set the second argument of AST_CXX_CHECK_STD to "yes",
the macro will automatically keep the highest `-std=gnu++` value that
worked and pass that to the Makefiles.
* The autoconf-archive package was added to install_prereq for future use.
* Updated configure.ac to use AST_CXX_CHECK_STD() to check for C++
versions 11, 14, 17, 20 and 23.
* Updated configure.ac to accept the `--enable-latest-cxx-std` option which
will set the second option to AST_CXX_CHECK_STD() to "yes". The default
is "no".
* ast_copy_string() in strings.h declares the 'sz' variable as volatile and
does an `sz--` on it later. C++20 no longer allows the `++` and `--`
increment and decrement operators to be used on variables declared as
volatile however so that was changed to `sz -= 1`.
(cherry picked from commit fb45a473cf)