Protect against unset variables [#79735118]
Wrapping vars in quotes when comparing
to avoid the "unary operator expected" warnings.
Allow 'symlinks' array in order to create link to deps

Use case:

* Handle multiple stacks:

Heroku is using the environment variable "STACK" in its buildpacks

(i.e. https://github.com/cloudfoundry/ruby-buildpack/blob/master/lib/language_pack/ruby.rb#L43-L45)

But quite a lot of files are common to different stacks and instead of
downloading a file several times, it could be interesting to do it
once, then create links.

Example:

```bash
symlinks = (
  'https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_ruby-2.1.0.tgz:https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_stackname_ruby-2.1.0.tgz'
  'https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_ruby-2.1.1.tgz:https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_stackname_ruby-2.1.1.tgz'
  'https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_ruby-2.1.2.tgz:https___s3-external-1.amazonaws.com_heroku-buildpack-ruby_stackname_ruby-2.1.2.tgz'
)
```