AX_CXX_COMPILE_STDCXX fails to select default C++ standard automatically

Hello,

I encountered an issue with |AX_CXX_COMPILE_STDCXX| (autoconf-archive) 
when used in a scenario where the user provides no C++ standard flags. 
The macro does not automatically select the highest available standard 
as expected. Instead, it may fail the build or require the user to 
manually specify |-std=c++XX|, which violates the principle that a 
configure script should select the best possible compiler options 
without manual intervention.

Steps to reproduce:

 1. Use a compiler supporting multiple C++ standards (e.g., gcc 5.x).
 2. Run |autoconf| / |./configure| without setting any |CXXFLAGS|
    including |-std|.
 3. Observe that the macro may fail or not choose |-std=c++11| (or the
    highest supported).

Expected behavior:

  * |AX_CXX_COMPILE_STDCXX| should automatically detect and use the
    highest supported standard if the user does not provide any flags.
  * The configure process should succeed without manual intervention.

Actual behavior:

  * The macro may require manual |-std| flags to proceed.
  * This behavior breaks the contract of configure scripts being able to
    configure software out-of-the-box.

Environment:

  * Autoconf version: 2.73
  * autoconf-archive version: latest
  * Compiler: GCC 5.5
  * OS: Solaris 10

This is a usability issue that can block clean builds in dual-toolchain 
or cross-platform scenarios. I suggest reviewing the default-selection 
logic to ensure the macro respects the “automatic best standard” 
principle when no user flags are set.

Thank you.