Prefer coarse timestamps when using X sync

They are good enough for this purpose, and are cheaper to get.
* src/timefns.c (monotonic_coarse_timespec): New function.
* src/xterm.c [HAVE_XSYNC && !USE_GTK && HAVE_CLOCK_GETTIME]:
(x_sync_current_monotonic_time): Use it.
(CLOCK_MONOTONIC): Remove; no longer uneeded here.

src/systime.h

@@ -79,6 +79,7 @@ enum { LO_TIME_BITS = 16 };
 
 /* defined in timefns.c */
 extern struct timeval make_timeval (struct timespec) ATTRIBUTE_CONST;
+extern struct timespec monotonic_coarse_timespec (void);
 extern Lisp_Object make_lisp_time (struct timespec);
 extern Lisp_Object timespec_to_lisp (struct timespec);
 extern struct timespec list4_to_timespec (Lisp_Object, Lisp_Object,

src/timefns.c

@@ -138,6 +138,26 @@ make_timeval (struct timespec t)
   return tv;
 }
 
+/* Return the current time with an epoch specific to this Emacs instance
+   (e.g., system boot).  The clock should be unaffected by changes to
+   the system time, and should be cheap to access.  Its resolution
+   should be appropriate for human time scales, e.g., better than 10 ms.
+   Make do with realtime if such a clock is not available.  */
+struct timespec
+monotonic_coarse_timespec (void)
+{
+  struct timespec ts;
+#ifdef CLOCK_MONOTONIC_COARSE
+  if (clock_gettime (CLOCK_MONOTONIC_COARSE, &ts) == 0)
+    return ts;
+#elif defined CLOCK_MONOTONIC
+  if (clock_gettime (CLOCK_MONOTONIC, &ts) == 0)
+    return ts;
+#endif
+  ts = current_timespec ();
+  return ts;
+}
+
 /* Yield A's UTC offset, or an unspecified value if unknown.  */
 static long int
 tm_gmtoff (struct tm *a)

src/xterm.c

@@ -7067,22 +7067,15 @@ x_sync_get_monotonic_time (struct x_display_info *dpyinfo,
   return ckd_sub (&t, timestamp, dpyinfo->server_time_offset) ? 0 : t;
 }
 
-# ifndef CLOCK_MONOTONIC
-#  define CLOCK_MONOTONIC CLOCK_REALTIME
-# endif
-
 /* Return the current monotonic time in the same format as a
    high-resolution server timestamp, or 0 if not available.  */
 
 static uint_fast64_t
 x_sync_current_monotonic_time (void)
 {
-  struct timespec time;
+  struct timespec time = monotonic_coarse_timespec ();
   uint_fast64_t t;
-  return (((clock_gettime (CLOCK_MONOTONIC, &time) != 0
+  return ((ckd_mul (&t, time.tv_sec, 1000000)
-	    && (CLOCK_MONOTONIC == CLOCK_REALTIME
-		|| clock_gettime (CLOCK_REALTIME, &time) != 0))
-	   || ckd_mul (&t, time.tv_sec, 1000000)
 	   || ckd_add (&t, t, time.tv_nsec / 1000))
 	  ? 0 : t);
 }