[C++] System Apache ORC used detection is broken

### Describe the bug, including details regarding any error messages, version, and platform.

https://github.com/apache/arrow/blob/8dd9eba99a69dbdf1d110cebf0bd349ee3ac7eda/cpp/src/arrow/CMakeLists.txt#L83

uses `ORC_SOURCE` but we define `orc_SOURCE`.

### Component(s)

C++