gfortran int8 problem

when we try to compile some preprocessed fortran
(Ingres esqlf) with gfortran we get an error that int8 is not an intrisic function.

"undefined reference to `int8_’ "

I have followed this through and see that int8 is an instrisic function in iso_fortran_env.

ISO_FORTRAN_ENV - The GNU Fortran Compiler

Can anyone suggest how to bind in the iso_fortran_env to the gfortran source or which command switch to use to use these intrinsic functions.

I have also attempted to ‘use’ the use statement
use :: iso_fortran_env within the fortran source but this fails to compile.

We are very new to Linux/Unix.

Can anyone suggest a more appropriate forum to post this question to?