We will continue to use the "normal" fish base directory detection when using
the CMake test harness which properly sets up a sandboxed $HOME for fish to use,
but when running source code tests with a bare `cargo test` we don't want to
write to the actual user's profile.
This also works around test failures when running `cargo test` under CI with a
locked-down $HOME directory (see #10474).
* Fix build on NetBSD
Notably:
1. A typo in `f_flag` vs `f_flags` - this was probably never tested
2. Some pointless name differences - `st_mtimensec` vs
`st_mtime_nsec`
3. The big one: This said that LC_GLOBAL_LOCALE() was -1 "everywhere".
Well, not on NetBSD.
* ifdef for macos
