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ARROW-7170: [C++] Fix linking with bundled ORC

Note I still get an error later:
```
[libprotobuf ERROR google/protobuf/descriptor_database.cc:120] File already exists in database: orc_proto.proto
[libprotobuf FATAL google/protobuf/descriptor.cc:1359] CHECK failed: GeneratedDatabase()->Add(encoded_file_descriptor, size):
terminate called after throwing an instance of 'google::protobuf::FatalException'
  what():  CHECK failed: GeneratedDatabase()->Add(encoded_file_descriptor, size):
Abandon
```

Closes #5834 from pitrou/ARROW-7170-orc-link-failure and squashes the following commits:

65c1c5d5d <Antoine Pitrou> ARROW-7170:  Fix linking with bundled ORC

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>