puzzled by 'cc1: error: unrecognized command line option "-fdefault-real-8"'

[Anthony Rollett]

Apologies if the solution to this is obvious but I have a macbook, with 10.6, up to date XCode, openmpi, gfortran (from http://r.research.att.com/tools/) installed.  Am trying to compile an mpi f90 program for which the options include -fdefault-real-8.  I get the error message in the subject line.  I tried using -r8 but I then get errors that suggest that real variables are being left as real*4, and not real*8.  If anyone can illuminate my ignorance I would be most grateful!
Tony Rollett, Carnegie Mellon