Updated dependencies for libstan

#### Summary:

Fixes dependencies for libstan and libstanc.

#### Intended Effect:

Fixes make behavior.

#### How to Verify:

Build libstan.a; change any of its dependencies; build again.
Under this branch, it rebuilds the lib. Under the old branch, this wasn't guaranteed.

#### Side Effects:

None.

#### Documentation:

Not user facing. Fixes a problem with our make.

#### Reviewer Suggestions:

Anyone.