If you use the old generic.cfg (which has "none" for both F77 and F90) and a thornlist that consists of the flesh only, Cactus will not compile. It will complain that CCTK_REAL is undefined.
Interestingly, if no definition is provided for F77 and F90 in the optionlist, then the Cactus build system assumes gfortran and compiles.
If, however, in addition to not defining F77 and F90 in the cfg, no fortran is actually present (i.e. not installed), then the CCTK_REAL issue returns.