build now handled by generated make rules
Significant improvement to build time w/ watchmedo

BuildKit launches an incremental modification tracking process based on
the watchmedo command (https://pypi.python.org/pypi/watchdog).
Instead of scanning the full source tree to detect modifications
in every modules on each build, it now incrementally updates the symlink
to the most recently modified file in each module.

After 30mins of idle activity, the monitor process will go away.  If
watchmedo isn't available, falls back to the previous behavior of full
scans.
+Portability: GNU stat -> Python snippet
Polished modification watcher

Shutdown whenever module is added/removed/moved.