Add build options for large external builds

[Paul-Ferrell]

This requires some discussion, but the jist is:

We need to point at builds that we don't actually want to copy.
I'm thinking this should have two components:

1. An option that says - hey, the build is over there, outside of Pavilion. Trust that it works.
2. An option that says - when you do the symlink copy, only go to depth X. At X - even directories should be symlinks to the original build (the current setup creates real directories with symlink to files in most cases).