Build should create directories as needed

[catamorphism]

There comes a time in the life of every Rust developer when they have to blow away various parts of the build directory to avoid messages about "multiple matching crates". Then when they do the next build they get things like: cp: x86_64-apple-darwin/stage1/lib/librustrt.dylib: No such file or directory. It would be better if the Makefile would just create all the needed parent directories automatically.

[brson]

+1

[pnkfelix]

I'm pretty sure I know how to do this. (I think the right tools for the job are some order-only prerequisites in host.mk and target.mk.)

[pnkfelix]

@catamorphism I haven't actually waited for a whole build to go through yet, and I have to leave for the evening. But I'd welcome it if you would be willing to pull this (SHA: 4dd0fa6) into your repo and give it a spin.

If you don't get around to it, no sweat; it can obviously wait until Monday.

[pnkfelix]

@catamorphism argh ignore comment above; the commit isn't anywhere near fully baked yet; somethings off with my make syntax.

[catamorphism]

@pnkfelix Okay. It would be nice to do this in the interim, but eventually when rustpkg is the build system it will do this (much more easily than with Makefile hacking), so no sweat if you don't get it done :-)