The troublesome behavior has been present for three years now and many projects have come to depend on it, thus a straight revert is not possible anymore.  However, #110286 is now being treated as a bug, not just a new feature request.

Effectively a duplicate of bug #110286 which I will treat as a priority item for 2.73.  No changes are going to be made for 2.72, however.

> * Noteworthy changes in release 2.70 (2020-12-08) [stable]

[...]

> * AC_PROG_CC now enables C2011 mode if the compiler supports it.
>
>   If not, it will fall back to C99 and C89, as before.  Similarly,
>   AC_PROG_CXX now enables C++2011 if available, falling back on C++98.

This breaks existing packages that have been relying on `AX_CXX_COMPILE_STDCXX` from the autoconf-archive package (<https://github.com/autoconf-archive/autoconf-archive/blob/master/m4/ax_cxx_compile_stdcxx.m4>) since about forever, because Autoconf itself did never care about the C++ standard version at all.

There appears to be no documentation on how to avoid the introduced behaviour, as far as I can see.

The new behaviour causes CXXFLAGS to look like '-std=gnu++11 -std=c++20', which is just confusing and arguably outright wrong.

In other cases, where the compiler default is actually newer than C++11 (e.g. GCC6 defaults to C++14, GCC11 defaults to C++17), it actively breaks compilation for programs that simply rely on the toolchain default.

See <https://github.com/macports/macports-ports/pull/17284#pullrequestreview-1415834125>.

Please revert this change.