>But as Christos said, we still need to explicitly list the
>dependencies in our Makefiles (LDADD, DPADD, ...) in the users of

Probably off topic, but for some time now I've used a dpadd.mk which
gets included by bsd.{prog,lib}.mk and handles lots of the grotty details
of this sort.  Eg. you can just do

DPLIBS+= ${LIBFOO} ${LIBGOO}

and dpadd.mk will ensure that DPADD and LDADD are updated accordingly.
This is perhaps more important in our build at work where we actually
never install anything anywhere - so dpadd also has to add things like:

LDADD+= ${DPADD:R:H:S/^/-L/}

and also adds appropriate -I's (that's a tad complex).

If you are considering an overhaul, this could be a useful addition.
There's a simple version of dpadd.mk included in pkgsrc/devel/mk-files

--sjg