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

[Kornel Benko]

commit a906676c8971fe4e5eb893937e8cba80cfe2cc12
Author: Kornel Benko <kornel at lyx.org>
Date:   Wed Oct 21 18:27:23 2020 +0200

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

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