Enable C++14 by default, because protobuf only supports C++14 and newer.

--- a/third_party/onnx/CMakeLists.txt.orig
+++ b/third_party/onnx/CMakeLists.txt
@@ -58,7 +58,7 @@
 # Required to use /std:c++17 or higher on Windows
 # For other platforms, set C++11 as standard for the whole project
 if(NOT MSVC)
-  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD 14)
 else()
   string(APPEND CMAKE_CXX_FLAGS " /std:c++17")
 endif()