Making it possible to have several concrete projects with different base PLTs

Previously you couldn't really have several concrete projects using different
`$(ERLANG_DIALYZER_APPS)' lists; or you had to nuke your  ~/.dialyzer_plt
file every time.

We now generate a base PLT ID that depends only on your
`$(ERLANG_DIALYZER_APPS)' list if you have either BSD's `md5' or OpenSSL's
`md5sum' around, otherwise we just take the project name as ID.