Backport a fix for AX_CXX_COMPILE_STDCXX

The serial 23 of this macro contained in the latest release has a bug. Detecting support for a required C++ standard may produce wrong results as alternatives aren't checked.

Before the inclusion of this patch:

checking whether g++ supports C++11 features by default... yes
checking whether g++ supports C++20 features by default... no
configure: error: *** A compiler with support for C++20 language features is required.

After:

checking whether g++ supports C++11 features by default... yes
checking whether g++ supports C++20 features by default... no
checking whether g++ supports C++20 features with -std=gnu++20... yes

And succesfull build of a project was possible, which requires C++20.
Interestingly this commit was made on the same day as the last release. I wonder if this should have been part of...