Fix depfiles compatibility with modern Automake.

Fix depfiles compatibility with modern Automake.

Automake 1.16 changes how depfiles are created.  It is now necessary
to define a rule which creates the stub and additionally we must clean
them explicitly.  This is a bit annoying to do but that's the price of
relying on undocumented internal behaviour, I guess.

The extra rule should be harmless when using Automake 1.15.