std::variant not found

## Problems summary
I want to use std::variant from c++17.
This however does not seem to work.
I always get "no template named 'variant' in namespace 'std'" when I run ALEDetail.

## Environment Information

- OS: OSX
- Neovim version: NVIM v0.3.1

## Provide a minimal init.vim with less than 50 lines and not plugin manager (Required!)

```vim
" Your minimal init.vim
Plug 'Shougo/deoplete.nvim'
Plug 'zchee/deoplete-clang'

let g:deoplete#sources#clang#std = 'c++17'
```