Avoid fdatasync() on Darwin. fsync() there doesn't actually flush writes to
storage like it does on Linux. So, use F_FULLFSYNC which was invented as an
alternative API to do this.
https://gitlab.com/drobilla/zix/-/issues/3
https://gitlab.com/drobilla/zix/-/commit/a6f804073de1f1e626464a9dd0a169fd3f69fdff
--- src/posix/filesystem_posix.c.orig
+++ src/posix/filesystem_posix.c
@@ -58,7 +58,13 @@ zix_get_block_size(const struct stat* const s1, const struct stat* const s2)
 static ZixStatus
 finish_copy(const int dst_fd, const int src_fd, const ZixStatus status)
 {
-  const ZixStatus st0 = zix_posix_status(dst_fd >= 0 ? fdatasync(dst_fd) : 0);
+#ifdef __APPLE__
+  const int rc = dst_fd >= 0 ? fcntl(dst_fd, F_FULLFSYNC) : 0;
+#else
+  const int rc = dst_fd >= 0 ? fdatasync(dst_fd) : 0;
+#endif
+
+  const ZixStatus st0 = zix_posix_status(rc);
   const ZixStatus st1 = zix_system_close_fds(dst_fd, src_fd);
 
   return status ? status : st0 ? st0 : st1;