[LyX/master] CMake: comment out c++20 for now

[Scott Kostyshak]

commit 2c1753a5265c59ebd11fc465cdb119ff589ad965
Author: Scott Kostyshak <skostysh at lyx.org>
Date:   Fri Oct 23 17:12:02 2020 -0400

    CMake: comment out c++20 for now
    
    We currently get errors with internal boost and also from LyX code.
---
 development/cmake/modules/FindCXX11Compiler.cmake |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/development/cmake/modules/FindCXX11Compiler.cmake b/development/cmake/modules/FindCXX11Compiler.cmake
index 7f22f59..f9a1aaf 100644
--- a/development/cmake/modules/FindCXX11Compiler.cmake
+++ b/development/cmake/modules/FindCXX11Compiler.cmake
@@ -54,7 +54,9 @@ else()
       endif()
     else()
       set(CXX11_FLAG_CANDIDATES
-        "--std=c++20"
+# Detection of c++20 works well, but our code is not ready for it yet.
+# We currently get errors with internal boost and also from our code.
+#        "--std=c++20"
         "--std=c++17"
         "--std=c++14"
         "--std=c++11"