chore(fs): build kqueue instead of fsevents watcher on iOS (#9950)
### Purpose
On iOS, the FSEvents API for watching files (also used on macOS) is not
available, but `kqueue` is. This PR ensures `kqueue` support is built on
iOS instead of the FSEvents based watcher implementation.
Before this PR, you could already use the `kqueue` build option to force
its usage. Unfortunately `gomobile`, the tool that I use to build
Syncthing for iOS and macOS for Synctrain, does not support setting
different build flags for iOS and macOS (unless I build separately for
each, which is a bit of a hassle because XCode nonsense). I am assuming
there are good reasons to support FSEvents even though `kqueue` is also
available on macOS (but I'm not sure why?). I do know FSEvents has been
working fine for me on macOS so it seems best to use FSEvents on macOS
and kqueue on iOS.
Note that this also requires https://github.com/syncthing/notify/pull/4
to be merged in `synchting/notify` (until that is done, this PR will
fail to build on iOS due to `notify` still trying to link to `fsevents`
stuff when the `kqueue` build flag is not set).
### Testing
I compiled both `syncthing/notify` and syncthing with this PR applied,
and used that to successfully build the Synctrain iOS app, which after
this PR works fine and should follow up file changes a bit quicker.
### Screenshots
n/a
### Documentation
n/a
## Authorship
Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.
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Co-authored-by: Jakob Borg <[email protected]>
Tommy van der Vorst