Add native node_monitor support on Haiku

[mmuman]

While fswatch builds fine on Haiku, it would be a shame not to use the node_monitor inherited from BeOS which predates inotify by at 10 or 15 years :smile:

This is mostly a reminder for myself, unless someone else wants to beat me at it.

Btw, the AC_CHECK_LIB([pthread]…) does work on Haiku even though linking with the lib is not required, but it'd probably be cleaner to use AX_PTHREAD instead since not all systems have the library, and some require a -pthread flag instead.