add example showing how to compile Cpp runtime with a different standard

runtime/Cpp/cmake/ExternalAntlr4Cpp.cmake

@@ -104,6 +104,7 @@ else()
       CMAKE_CACHE_ARGS
           -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
           -DWITH_STATIC_CRT:BOOL=${ANTLR4_WITH_STATIC_CRT}
+          # -DCMAKE_CXX_STANDARD:STRING=17 # if desired, compile the runtime with a different C++ standard
       INSTALL_COMMAND ""
       EXCLUDE_FROM_ALL 1)
 endif()