[LyX/master] Revert "cmake build: Allow use of '--std=c++17' for gnu compiler"

[Kornel Benko]

commit d3c16d049697b05590b936db1219d5eb51cb4294
Author: Kornel Benko <kornel at lyx.org>
Date:   Thu Oct 22 12:04:41 2020 +0200

    Revert "cmake build: Allow use of '--std=c++17' for gnu compiler"
    
    This reverts commit a906676c8971fe4e5eb893937e8cba80cfe2cc12.
---
 development/cmake/modules/FindCXX11Compiler.cmake |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/development/cmake/modules/FindCXX11Compiler.cmake b/development/cmake/modules/FindCXX11Compiler.cmake
index 96b72eb..9188ab6 100644
--- a/development/cmake/modules/FindCXX11Compiler.cmake
+++ b/development/cmake/modules/FindCXX11Compiler.cmake
@@ -54,7 +54,6 @@ else()
       endif()
     else()
       set(CXX11_FLAG_CANDIDATES
-        "--std=c++17"
         "--std=c++14"
         "--std=c++11"
         "--std=gnu++11"