timespec_get() and TIME_UTC are C11 and only reached macOS in 10.15, so qemu
11.1.0 fails on 10.14 and earlier.

clock_gettime() has been available since 10.12 and TIME_UTC designates the
realtime clock, so CLOCK_REALTIME is the direct equivalent.  The guard tests
MAC_OS_X_VERSION_MIN_REQUIRED so that building against a newer SDK while
targeting 10.14 still takes the fallback.

--- contrib/plugins/uftrace.c.orig
+++ contrib/plugins/uftrace.c
@@ -144,7 +144,14 @@
 #else
     /* We need nanosecond precision for short lived functions. */
     struct timespec ts;
+    /* timespec_get() is C11 and only reached macOS in 10.15; older SDKs
+     * provide clock_gettime(), for which TIME_UTC's clock is CLOCK_REALTIME. */
+#if defined(__APPLE__) && (!defined(MAC_OS_X_VERSION_10_15) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_15)
+    clock_gettime(CLOCK_REALTIME, &ts);
+#else
     timespec_get(&ts, TIME_UTC);
+#endif
     uint64_t now_ns = ts.tv_sec * NANOSECONDS_PER_SECOND + ts.tv_nsec;
 #endif
     return now_ns;