I'm pretty sure this must be the intended behaviour. It's kind of
meaningless to use this to declare a dependency on a file that might start
to exist later, if the file already exists.
'./redo -d test' reveals that we check and re-check things like CC and LD
even when they're children of objects that don't need to be rebuilt, and who
we've already checked earlier in the build cycle. We need to be a little
smarter about pruning if we want dependency checking to be fast.
And move test scripts into t/ subdir to keep things clean.
As part of that, make sure redo adds itself to the PATH on startup so that
subscripts can find it.
