gfortan 9 produces warnings in sibyll

With gfortan 9, I get warnings like

/corsika/corsika/Processes/Sibyll/sibyll2.3c.f:6655:72:

 6655 | 250     RORD(IL2+1)=RORD(IL2)
      |                                                                        1
Warning: Fortran 2018 deleted feature: DO termination statement which is not END DO or CONTINUE with label 250 at (1)

I tried selecting the fortran language standard using the -std flag to fortran. I tried the allowed values ‘f95’, ‘f2003’, ‘f2008’, ‘f2018’, ‘gnu’, or ‘legacy’. Only the last value, 'legacy', will let me compile w/o error and w/o warnings. Using 'gnu' works with warnings. Selecting a strict compliant standard (‘f95’, ‘f2003’, ‘f2008’, or ‘f2018’) leads to an error that the code uses gnu extensions. @riehn: we should report this to the Sibyll developers.

Using 'legacy' seems to be the right choice for use with CORSIKA.