MOOG 2019 and gfortran
The SM developers seem to have anticipated such issues.
In your nm output sample,
T sm_ylabel is the native C function, and
T sm_ylabel_ is its wrapper to be called from Fortran.
gfortran usually appends single underscores to external symbol names.
If the MOOG object files were in fact built with the gfortran options in the question, the symbol references should match the wrappers in the SM library.
The double underscore suffixes may be left over from a prior build attempt with -ff2c.
When you change compiler options, remember to make clean before rebuilding.
If that doesn't lead to a solution, and nothing else is using SM,
the SM manual suggests
that you could rebuild it with this in its options.h:
#define FORTRAN_APPEND __