I think that the warning messages should not be displayed -

[dranta:~/tests/gfortran-D] dir% gfortran -g -c -fdefault-real-8
-fdefault-integer-8 -fdefault-double-8 arg.F
cc1: warning: command line option "-fdefault-real-8" is valid for F95 but not for C
cc1: warning: command line option "-fdefault-integer-8" is valid for F95 but not
for C
cc1: warning: command line option "-fdefault-double-8" is valid for F95 but not
for C
[dranta:~/tests/gfortran-D] dir% cat arg.F
      program main
      stop
      end