[RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15)

[Florian Weimer]

* Mathieu Desnoyers via Libc-alpha:

> Changes since v14:
> - Update copyright range to include 2020.
> - Introduce __ASSUME_RSEQ defined for --enable-kernel=4.18.0 and higher.
> - Use ifdef __ASSUME_RSEQ rather than ifdef __NR_rseq to discover rseq
>   availability. This is necessary now that the system call numbers are
>   integrated within glibc.

It's not quite clear to me why you need __ASSUME_RSEQ.

Can you use __has_include in <sys/rseq.h>, with a copy of the kernel
definitions if the kernel header is not available?