I need to use svn for some dependencies, or even a directory not under source control

I've changed deps.mk to add svn and present as $$$$VS values.
Because svn specifies branch as part of the url, I've required $$$$COMMIT be just an unquoted hyphen ("-").
Because an already present directory has no branch or commit, I've required $$$$REPO and $$$$COMMIT both be an unquoted hyphen.

Additionally, I've changed erlang.mk as built using the new deps.mk.

I'll be happy to adjust as needed for acceptance.