Bug#900447: Build against freerdp2

[Fabián Rodríguez]

I've asked upstream to clarify if Guacamole v 1.0.0 now supports FreeRDP
v2, last update there was in September:

https://issues.apache.org/jira/browse/GUACAMOLE-249

The answer for now is "no", so I agree dropping only RDP for now would
help, at least enabling publishing a new version in Debian as it's
nearing feature/release freeze in ~1 month.