Add dependency tracking (makedep)

This is a very large change of a central part of Erlang.mk.
I will admit that I am not quite confident on that one. If
you do have issues following this change, please open a
ticket and I will look at it immediately.

At this point, it works for me, but I wouldn't be surprised
to hear about a few minor issues.

This commit introduces a dependency file $(PROJECT).d which
contains Makefile rules between Erlang source files and
headers, behaviors and parse_transforms. This allows us
to rebuild only the files that are needed.

The $(PROJECT).d is generated automatically when missing,
and when any of the files change.

It is possible to hook before and after this generation,
by defining a $(PROJECT).d:: rule. This allows users to
generate Erlang files which are then compiled by Erlang.mk
automatically (and to track their dependencies, of course).

Here goes nothing...

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