GUACAMOLE-249: Allow detected FreeRDP plugin directory to be overridden with --with-freerdp-plugin-dir.
GUACAMOLE-249: Allow detected FreeRDP plugin directory to be overridden with --with-freerdp-plugin-dir.

glyptodon/guacamole-server,glyptodon/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,glyptodon/guacamole-server