AC_PROG_CXX now tries C++23, C++20, C++17, C++14

AC_PROG_CXX now tries C++23, C++20, C++17, C++14

The recent glitch with C++11 underscores the fact that it’s
long past time for Autoconf to catch up with the C++ standards.
Add simple tests for each iteration of the C++ standard.
They can be improved later as needed.  When testing for these
newer standards, do not also test for features required by earlier
standards, as this isn’t all that helpful, leads to an O(N**2)
growth of ‘configure’, and can result in problems such as the
C++11 vs C++20 bug recently fixed.
* lib/autoconf/c.m4 (_AC_CXX_CXX14_TEST_PROGRAM)
(_AC_CXX_CXX17_TEST_PROGRAM, _AC_CXX_CXX20_TEST_PROGRAM)
(_AC_CXX_CXX23_TEST_PROGRAM, _AC_CXX_CXX14_OPTIONS)
(_AC_CXX_CXX17_OPTIONS, _AC_CXX_CXX20_OPTIONS)
(_AC_CXX_CXX23_OPTIONS): New macros.
(_AC_C_C23_TEST_PROGRAM, _AC_C_C23_OPTIONS): New macros.
(_AC_PROG_CC_STDC_EDITION): Try C23 first.
(_AC_PROG_CXX_STDCXX_EDITION): Use them.