macos: Compilation issue with missing `fdatasync`

[cdecker]

Issue and Steps to Reproduce

I'm trying to compile master @ 620a61e on MacOS via a nix-shell, as that appears to be the most reliable way of creating reproducible builds on MacOS. However, it appears that the recent dynamic configs PR #6303 added a call to fdatasync which may not be available.

Would using fsync be a good alternative? According to the man-pages it is equivalent, but also syncs metadata, so causes more write operations.