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)
```
Add the sshagent plugin to forward the ssh-agent protocol over an RDP
dynamic virtual channel, just as the normal ssh-agent forwards it over
an SSH channel. Add the "/ssh-agent" command line option to enable it.
Usage:
Run FreeRDP with the ssh-agent plugin enabled:
xfreerdp /ssh-agent ...
In the remote desktop session run xrdp-ssh-agent and evaluate the output
in the shell as for ssh-agent to set the required environment variables
(specifically $SSH_AUTH_SOCK):
eval "$(xrdp-ssh-agent -s)"
This is the same as for the normal ssh-agent. You would typically do
this in your Xsession or /etc/xrdp/startwm.sh.
Limitations:
1. Error checking and handling could be improved.
2. This is only tested on Linux and will only work on systems where
clients talk to the ssh-agent via Unix domain sockets. It won't
currently work on Windows but it could be ported.
