Separate target configuration output directories

[parkovski]

#4427 related. I use the Linux subsystem on Windows 10 and commonly build a Windows and Linux build from the same project folder - currently both binaries end up in target/debug. It'd be nice to have the option to set up target-based outputs, so I could get Windows binaries in target/windows/debug and Linux in target/linux/debug.

My preference for this would be providing some interpolated variables so I could put something like this in my Cargo.toml:

output-dir = "target/${target.os}/${profile}"

[sfackler]

This will happen if you pass --target explicitly.

[alexcrichton]

Thanks for the report! As @sfackler notes, however, this already happens when you pass --target