In SoftSerial.h and SoftSerial.cpp find the following:
virtual void SoftwareSerial::write(uint8_t)
and/or
void SoftwareSerial::write(uint8_t)
and change it to:
virtual size_t SoftwareSerial::write(uint8_t)
and/or
size_t SoftwareSerial::write(uint8_t)
in the CPP file, you will also have to add to the end of the function mentioned above:
return 1;