Auto merge of #4797 - lukaslueg:issue3169, r=alexcrichton

Bail out when trying to link to a library that is not linkable.

There are more subtleties here than expected, as we can have situations where it is actually Ok to have no linkable targets: Build scripts are a common case, yet benchmark tests started to also fail. I have to say I'm not convinced if the situation "not one target is linkable, yet at least one target is a library (and therefor at least something should be linked)" is actually correct. All tests pass, however, including the one that checks for #3169