Problem creating S-Function

I'm trying to buyild a S-Function from a FORTRAN 90 program that is originally compiled against the Intel FORTRAN compiler. I modified the code now so that it compiles now with MinGW and gfortran on Windows. Now I want to create a S-Function of this program to use in MATLAB/Simulink. There is already mex file made for the Intel FORTRAN compiler. I changes that file now to work with gnumex and the gfortran compiler. But I am get errors about missing MATLAB routines mexPrintf and mexErrMsgTxt, e.g.

undefined reference to `mexprintf'

I know this is caused by case sensitivity being ignored in fortran. How do I fix this?