These are generated in many different ways in the various
subdirectories, and sometimes not generated correctly. Introduce a
script to do this in one place, and hopefully correctly.
This is mostly simple refactoring, but I got rid of a couple of
things:
(1) The ‘make depend’ rule doesn't appear to be needed. automake (or
make?) seems to rebuild the ‘.depend’ file automatically just because
it is included.
(2) I got rid of the hairy path rewriting sed expression. Possibly
that is needed for srcdir != builddir.
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