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

Making it possible to have concrete projects with different base PLTs

Previously you couldn't really have several concrete projects using different
`$(ERLANG_DIALYZER_APPS)` lists, nor distinct erlang versions; 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 and erlang version if you have either BSD's
`md5` or OpenSSL's `md5sum` around, otherwise we just rely on the project name
and erlang version to generate the ID.