[mm-common/c++20] wip: MM_AX_CXX_COMPILE_STDCXX(): Allow 20 as a possible value

[Murray Cumming <murrayc src gnome org>]

commit b63fee88f7fc7cd9cd28f2e83f938b0fdbe90e69
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Oct 27 14:51:44 2019 +0100

    wip: MM_AX_CXX_COMPILE_STDCXX(): Allow 20 as a possible value
    
    By using the --std=2a option available with recent g++ and clang++
    versions.

 macros/mm-ax_cxx_compile_stdcxx.m4 | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/macros/mm-ax_cxx_compile_stdcxx.m4 b/macros/mm-ax_cxx_compile_stdcxx.m4
index ebd372f..e93067d 100644
--- a/macros/mm-ax_cxx_compile_stdcxx.m4
+++ b/macros/mm-ax_cxx_compile_stdcxx.m4
@@ -53,6 +53,7 @@ AC_DEFUN([MM_AX_CXX_COMPILE_STDCXX], [dnl
   m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
+        [$1], [20], [ax_cxx_compile_alternatives="20 2a"],
         [m4_fatal([invalid first argument `$1' to MM_AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [ext], [],
@@ -157,6 +158,11 @@ m4_define([_MM_AX_CXX_COMPILE_STDCXX_testbody_17],
   _MM_AX_CXX_COMPILE_STDCXX_testbody_new_in_17
 )
 
+m4_define([_MM_AX_CXX_COMPILE_STDCXX_testbody_20],
+  _MM_AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+  // TODO
+)
+
 dnl  Tests for new features in C++11
 
 m4_define([_MM_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[