undefined reference when using gfortran to compile fortran 77 code

undefined reference when using gfortran to compile fortran 77 code

I have an old fortran 77 code, which compiles well on Sun workstation. However, there are errors when I am trying to compile it using gfortran on ubuntu(64 bits): How to solve this problem? thank you very much.
$ gfortran -Wall -g -Jobj/Debug/ -ffixed-form -lc -lm -lg2c -c main.for -o obj/Debug/main.o
The following is the information given by gfortran
3015 main.for|32994| undefined reference to `qextd_'
3016 main.for|33000| undefined reference to `qextd_'
3017 main.for|33000| undefined reference to `qmod_'
3018 main.for|33000| undefined reference to `dbleq_'
3019 main.for|33015| undefined reference to `dbleq_'
3020 main.for|33015| undefined reference to `dbleq_'