FindProtobuf: Imported targets do not link to threads
Protobuf headers have dependencies on threads. On UNIX platforms this requires linking to a threads library. This was added to FindProtobuf long ago by commit 8961c4b6, but that dependency is not expressed in the
protobuf:: imported targets added by !861 (merged). This did not seem to matter until protobuf 3.6 started throwing an exception when not linked to threads.