I believe the trouble is the SoftwareSerial library was written for the earlier versions of Arduino, such as the 0.22 release. In that version, the write function did not return anything (i.e. it had a void return). In the 1.0 release and beyond, write now returns the amount of bytes that were actually written (i.e. the return type is size_t).
You would presumably need to modify the .h file to change the calling signature to size_t write (...), and then change the .cpp file to return a 1 in the write functions.
I would imagine that somebody else has already done this, and you might be able to find a newer version of SoftwareSerial.