+ <http://tangent.org/552/libmemcached.html>
+
+ * libmodbus (optional)
+ Used by the “modbus” plugin to communicate with Modbus/TCP devices. The
+ “modbus” plugin works with version 2.0.3 of the library – due to frequent
+ API changes other versions may or may not compile cleanly.
+ <http://www.libmodbus.org/>