Part of #447.
* configure.ac, m4/debug.m4, m4/devel.m4, m4/gccoptim.m4,
m4/ndebug.m4: Replace AC_ERROR and AC_HELP_STRING by
by AC_MSG_ERROR and AS_HELP_STRING.
Part of issue #447.
Autoconf 2.69 was released in 2012, so it now widely available. The
recent release of 2.70 is obsoleting some constructs, so it will be
easier on us if we do not have too many versions to support.
* HACKING, configure.ac: Require Autoconf 2.69.
Part of #447.
* configure.ac, m4/debug.m4, m4/devel.m4, m4/gccoptim.m4,
m4/ndebug.m4: Replace AC_ERROR and AC_HELP_STRING by
by AC_MSG_ERROR and AS_HELP_STRING.
Part of issue #447.
Autoconf 2.69 was released in 2012, so it now widely available. The
recent release of 2.70 is obsoleting some constructs, so it will be
easier on us if we do not have too many versions to support.
* HACKING, configure.ac: Require Autoconf 2.69.
* configure.ac: Call CHECK_ATOMIC after setting the C++ options.
This is probably not necessary, but it makes more sense.
* m4/l_atomic.m4: Augment the test body with a scenario
that actually requires -latomic.
* configure.ac: Activate C++17, and replace --enable-c++17 by
--enable-c++20.
* NEWS: Mention the news.
* .gitlab-ci.yml: Use C++20 for the former C++17 builds.
* HACKING, README, doc/org/compile.org, doc/org/concepts.org,
doc/org/index.org, doc/org/install.org, doc/org/tut.org,
doc/org/upgrade2.org, spot/misc/escape.hh: Adjust mentions
of C++14.
