[LyX/master] Autotools: use C++17 mode when available

[Jean-Marc Lasgouttes]

commit bf28a2677c2ebfbaef9cffbb240aa886b9f8cbbc
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Thu Dec 10 10:55:09 2020 +0100

    Autotools: use C++17 mode when available
---
 config/lyxinclude.m4 |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/config/lyxinclude.m4 b/config/lyxinclude.m4
index a3bb114..d07e1bc 100644
--- a/config/lyxinclude.m4
+++ b/config/lyxinclude.m4
@@ -270,8 +270,8 @@ AC_REQUIRE([AC_PROG_CXXCPP])
 
 ### We might want to force the C++ standard.
 AC_ARG_ENABLE(cxx-mode,
-  AS_HELP_STRING([--enable-cxx-mode],[choose C++ standard (default: 14, then 11)]),,
-  [enable_cxx_mode={14,11}]
+  AS_HELP_STRING([--enable-cxx-mode],[choose C++ standard (default: 17, 14, then 11)]),,
+  [enable_cxx_mode={17,14,11}]
 )
 
 AC_LANG_PUSH(C++)