Improve flexibility of dependencies' locations on OS X

This PR might break the installation procedure of bindings (especially if it involves using `install_name_tool` to "fix" paths). But it should not be insurmountable.

Anyway, this fixes `feature/unittest` and is required for working on `feature/no_libsndfile`.
Yep, I prefer to separate each <key> visually if they are not directly related to each other. :-)