[LyX/master] Fix search for a good c++ compiler

[Jean-Marc Lasgouttes]

commit 2c694ccc2b478fca27503aff11657e03c87f9499
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Mon Nov 4 11:58:52 2024 +0100

    Fix search for a good c++ compiler
    
    Bash evaluates "c++{17,14}" as "c++17 c++14", but "c++-{17}" as "c++{17}".
---
 config/lyxinclude.m4 | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/config/lyxinclude.m4 b/config/lyxinclude.m4
index 154fe16072..d7063c8ec8 100644
--- a/config/lyxinclude.m4
+++ b/config/lyxinclude.m4
@@ -232,7 +232,8 @@ 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: 17)]),,
-  [enable_cxx_mode={17}]
+  dnl put modes in curly braces if there are several of them (ex. {20,17}
+  [enable_cxx_mode=17]
 )
 
 AC_LANG_PUSH(C++)