Create c11/time.h instead of put timespec_get in `c11/threads.h`
Creating impl folder is used to avoid `#include <time.h>` point the c11/time.h file
Detecting if `struct timespec` present with meson
Define TIME_UTC in `c11/time.h` instead `c11/threads.h`
Define `struct timespec` in `c11/time.h` when not present.
Implement timespec_get in c11/impl/time.c instead threads.h
Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15497>
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