c11: Implement c11/time.h with c11/impl/time.c

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>

include/c11/threads.h

@@ -29,11 +29,7 @@
 #ifndef EMULATED_THREADS_H_INCLUDED_
 #define EMULATED_THREADS_H_INCLUDED_
 
-#include <time.h>
-
-#ifndef TIME_UTC
-#define TIME_UTC 1
-#endif
+#include "c11/time.h"
 
 /*---------------------------- types ----------------------------*/
 typedef void (*tss_dtor_t)(void*);