CMake v3.5.1 throws the following error if this is not set:
```
CMake Error at channels/CMakeLists.txt:48 (if):
if given arguments:
"OFF" "OR"
Unknown arguments specified
Call Stack (most recent call first):
channels/sshagent/ChannelOptions.cmake:6 (define_channel_options)
channels/CMakeLists.txt:273 (include)
```
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