After playing around with the settings a bit it seems the error arose because I set the OpenMP flag in BLASOPT, which is not invoked when compiling ccsd_t_kernels_omp.F. It seems the correct approach is to use the option USE_OPENMP=y, which adds -qopenmp to the general compilation flags, and then it compiles without problems.
Yes, that's the most appropriate way to use OpenMP constructs in NWChem