[DirectoryWatcher] Do not use FSEvents on non-macOS platforms

The FSEvents APIs are available on iOS6+: however, the DirectoryWatcher
code isn't wired up to really use FSEvents on embedded platforms.

I've duplicated code from DirectoryWatcher-not-implemented.cpp here and
used TargetConditionals instead of adding cmakery to check try_compile;
I couldn't get that to work properly.