[PATCH] Add support for C++2a stop_token

[Thomas Rodgers]

Tested x86_64-pc-linux-gnu, committed to trunk.

Jonathan Wakely writes:

> On 13/11/19 17:59 -0800, Thomas Rodgers wrote:
>>+/** @file include/stop_token
>>+ *  This is a Standard C++ Library header.
>>+ */
>>+
>>+#ifndef _GLIBCXX_STOP_TOKEN
>>+#define _GLIBCXX_STOP_TOKEN
>>+
>>+#if __cplusplus >= 201703L
>
> This should be > not >=
>
> OK for trunk with that change.