[PATCH] A steadier steady_clock

[Florian Weimer]

* Sam Varshavchik:

> Based on a casual browsing of clock_gettime(3), CLOCK_MONOTONIC_RAW
> seems to be a better fit for std::chrono::steady_clock's requirements
> as given in  20.11.7.2, with recent Linux kernels,

Are the Linux clock semantics documented somewhere in detail?

> +#ifdef CLOCK_MONOTONIC_RAW
> +      clock_gettime(CLOCK_MONOTONIC_RAW, &tp);
> +#else
>       clock_gettime(CLOCK_MONOTONIC, &tp);
> +#endif

If a #define is available at compile time, it's not necessarily the
case that the feature is present at run time.  If CLOCK_MONOTONIC_RAW
is indeed what we want, we need fallback code.