No special options are required, but some settings in eclipse and the general project setup are critical. Declaring HardwareSerial extern or some such thing is definitely not required, you must have a broken setup.
Look here for a detailed explanation:
http://www.arduino.cc/playground/Code/Eclipse
Be aware that there are quite a few bugs in the eclipse AVR plugin, and some in the eclipse CDT plugin which the AVR one is based on.