It's useful to have a standard way to clean up the work done by the
depends system when testing changes to it.

The `make clean-all` rule removes build artifacts for all
supported architectures (in addition to sources/), while `make clean`
only removes artifacts for current architecture (`BUILD`).

Backport of core PR12715.

Copy your depends outside of your source tree and cd to this dir.

Should remove you current arch directory, the work and built
subdirs:

make clean

Note: my system was detected as x86_64-pc-linux-gnu despite the triple
for the depends was x86_64-linux-gnu so I had to rename it.

Should remove the directories for all architectures and the sources
subdir:

make clean-all