UUID: fix seeding the RNG clock on macOS

On Apple machines, call `clock_gettime()` instead of `timespec_get()`.

macOS only introduced `timespec_get()` in version 10.15 (introduced
approx two years ago, so in 2019), even though the function is from C11.

source/blender/blenlib/intern/uuid.cc

@@ -39,7 +39,16 @@ UUID BLI_uuid_generate_random()
     static_assert(std::mt19937_64::max() == 0xffffffffffffffffLL);
 
     struct timespec ts;
+#ifdef __APPLE__
+    /* `timespec_get()` is only available on macOS 10.15+, so until that's the minimum version
+     * supported by Blender, use another function to get the timespec.
+     *
+     * `clock_gettime()` is only available on POSIX, so not on Windows; Linux uses the newer C++11
+     * function `timespec_get()` as well. */
+    clock_gettime(CLOCK_REALTIME, &ts);
+#else
     timespec_get(&ts, TIME_UTC);
+#endif
     /* XOR the nanosecond and second fields, just in case the clock only has seconds resolution. */
     uint64_t seed = ts.tv_nsec;
     seed ^= ts.tv_sec;