This is a departure from how djb seems to have it set up, but I just like it
better. It's more like the reasonably-common Makefile standard. (Although
what make *actually* does is just use the first target declared in the
file.)
This is what GNU make does. If SHELL isn't defined, we still fall back to
calling sh.
Rumour has it that Google has some kind of build system that can be
massively distributed if you just set SHELL to the right program; maybe
it'll work with redo now. (Of course it won't do you any good until we
implement parallel builds...)
redo-ifchange already does this, and then calls redo.py, so it's redundant.
The only weird case is if a .do file calls 'redo' instead of
'redo-ifchange'. I'm not sure what exactly is supposed to happen here.
test.do does this, for example, and it works, but only because we always do
'redo test' and nobody depends on 'redo-ifchange test'; if they did, the
dependencies would be off, I guess. But maybe it's meaningless to do that
anyway.
So if we're in redo/t/ and running 'redo hello', we'll detect redo/.redo and
use that one rather than creating a new redo/t/.redo directory.
The downside of this is we get slightly different behaviour if the *first*
thing you build isn't from the root. Probably that's bad, but it should
hopefully be rare.
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.
