We have to avoid preprocessor macros spaghetti, like for various OS approaches regarding extended attributes.
We already have NO_XATTR for systems not supporting it, but if they support it differently than Linux, then common OS-agnostic API should be introduced internally that would have OS-specific implementations for different POSIX systems.

Let's not tackle with non-POSIX systems like Windows, as it would require even bigger changes, that may not be really suitable for metastore.

There were some existing efforts to make metastore work on systems different than Linux.

• macOS
  Support building on macOS #48, see also comments in Mac support #55
• FreeBSD
  FreeBSD support attempt #60, see also discussion in FreeBSD support #54
• OpenBSD
  Add support for OpenBSD. #43, see also discussion in Mac support #55
• Cygwin (Windows)
  Cygwin and Mingw support attempt #61, see also discussion in Cygwin and Mingw (Windows) support #56