cargo clean hook to clean up arbitrary stuff.

[vext01]

Following on from #5077, we had a project which built some C code using an external build system (make in this case, but it could be autotools or cmake for example):
https://github.com/vext01/rust_link_test

Currently there is no way to clean the externally built code at cargo clean time. Such a feature would be very useful.

[sfackler]

If you run the build in OUT_DIR, it'll be cleaned up via a cargo clean.

[alexcrichton]

I think this is a dupe of #572, so I'm going to close in favor of that

[vext01]

Thanks