[glibc] <sys/stat.h>: Use Linux UAPI header for statx if available and useful

[Joseph Myers]

On Wed, 12 Jun 2019, Zack Weinberg wrote:

> I wonder how much stuff still depends on these legacy user-namespace
> OS-specific macros.  Would anyone even notice if we put
> 
> #undef linux
> #undef unix
> 
> at the top of features.h?

That is of course invalid for anyone using -std= to prevent the compiler 
defining such macros and then defining their own "linux" or "unix" macros 
before including standard headers.

-- 
Joseph S. Myers
joseph@codesourcery.com