which checks if $CXX is a C++11 compiler and if not it tries with
switches like -std=c++11. The noext macro option means that no GNU
extension is looked for, i.e., -std=c++11 is tried rather than -std=gnu++11.
This works fine and desired with autoconf 2.69 and CXX is set to 'g++
-std=c++11'. If I read the code of Autoconf 2.70 correctly, it seem not
trivial to get the same behaviour because AC_PROG_CXX sets CXX to 'g++
-std=gnu++11', and then when AX_CXX_COMPILE_STDCXX kicks in CXX is
already a c++11 compiler, so it does not alter anything. Will there be a
way to avoid the GNU extensions to be turned on?