Accept `$(force_rebuilding_dep)` to tune the behavior of `$(FULL)`

By default, Erlang.mk only builds dependencies once, the first time the
top-level project is compiled. A forced rebuild of all dependencies can
be forced by setting the `$(FULL)` variable to a non-empty value.

For developers working on the top-level project and one or more
dependencies at the same time, Erlang.mk now accepts a variable/function
called `$(force_rebuilding_dep)` which is called to determine if the
dependency passed as the first argument should be forcibly rebuilt.

It allows a developer to filter the dependencies he works on (so they
are always rebuilt) from other third-party dependencies he never touches
(i.e. they are only built once at the beginning).

The content of `$(force_rebuilding_dep)` is expected to be a shell
one-liner where the `$(1)` Make variable is set to the path to the
dependency.

Here is an example from the testsuite:

    force_rebuilding_dep = test '$(1)' = '$(CURDIR)/deps/cowlib'

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