Specify output directory for binary executables

I was getting annoyed having to add a new entry to my project's `.gitignore` whenever I produced a new executable. This change adds a `RUSTBINDIR` variable (set to `bin` by default) which allows the user to place the executable outputs of crates in their own directory. Installed executables are still always placed in `$$(RUSTINSTALLDIR)/bin`.

If you'd like default behavior to remain the same I can change the default to `.`, but I think this is nicer.