You can use your standard Fortran compiler to build this program. Some examples are gfortran or gcc. Using gfortran is recommended.
Run
gfortran thxgen.f95 -o thxgenAlthough I can't guarantee it'll work - Fortran is all old and creaky.
I'm really excited about this because last time I tried to make a version of thxgen in Fortran, I couldn't do it.