From df4915dec4b84a09745b6dcd9d9496f1f7313d4b Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Sun, 15 Oct 2017 21:32:10 -0700
Subject: [PATCH] m4: Add missing C++17 related stub and possibly helpful
 message.

---
 m4/ax_cxx_compile_stdcxx.m4 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/m4/ax_cxx_compile_stdcxx.m4 b/m4/ax_cxx_compile_stdcxx.m4
index 95da0e321..14d34c888 100644
--- a/m4/ax_cxx_compile_stdcxx.m4
+++ b/m4/ax_cxx_compile_stdcxx.m4
@@ -47,7 +47,7 @@ dnl  (serial version number 13).
 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
   m4_if([$1], [11], [],
         [$1], [14], [],
-        [$1], [17], [m4_fatal([support for C++17 not yet implemented in AX_CXX_COMPILE_STDCXX])],
+        [$1], [17], [],
         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [gnu], [],
@@ -119,7 +119,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
   AC_LANG_POP([C++])
   if test x$ax_cxx_compile_cxx$1_required = xtrue; then
     if test x$ac_success = xno; then
-      AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
+      AC_MSG_ERROR([*** A compiler with support for C++$1 with SD-6 $4 language features is required. Ubuntu users may want to try: sudo add-apt-repository ppa:ubuntu-toolchain-r/test; sudo apt-get update])
     fi
   fi
   if test x$ac_success = xno; then
@@ -149,6 +149,14 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
 )
 
 
+dnl  Test body for checking C++17 support
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11($1)
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14($1)
+)
+
+
 dnl  Tests for new features in C++11
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[