Makes the build runner compile successfully for non-linux targets;
printing an error if you ask for --watch rather than making build
scripts fail to compile.
And use it to implement InstallDir Step watch integration.
I'm not seeing any events triggered when I run `mkdir` in the watched
directory, however, and I have not yet figured out why.
So far, only implemented for InstallFile steps.
Default debounce interval bumped to 50ms. I think it should be
configurable.
Next I have an idea to simplify the fanotify implementation, but other
OS implementations might want to refer back to this commit before I make
those changes.
