Set ERL_LIBS to $(DEPS_DIR) rather than `deps`

This solves a recursion problem when compiling deeper recursive
makefile structures. All dependencies are pulled to the top-level
dependencies directory by virtue of DEPS_DIR, but the recursive make
calls to erlc fails since the value is hard-coded.
Merge branch 'fix-recursive-erlc-call' of git://github.com/jlouis/erlang.mk