r22441: Add automatic dependency tracking. The dependency generation is

r22441: Add automatic dependency tracking. The dependency generation is
toolchain independent, but since there's no portable method for
doing makefile includes, we can actually use the dependencies with
GNU make. It's easy to add this for other makes, but I don't have
any to test.

This also moves as much m4 as possible into the m4 directory where
no-one has to look at it. AFAICT, there's no way to hide depcomp
in there too, which is unfortunate.
(This used to be commit aa14900f8291a017aa7fab2bbb9a6c79b12889b2)