Clima incompatible with gfortran-8

[will-fawcett]

When compiling Clima with the gfortran-8 (packaged with gcc-8) the following compilation error occours:

CLIMA/RRTM/rrtatm.f:878:38:

                         CALL CMPALT(2,PTMP,TTMP,
                                      1
Error: Actual argument contains too few elements for dummy argument 'pm' (2/3400) at (1)
CLIMA/RRTM/rrtatm.f:943:32:

which is repeated for all of the calls of CMPALT in rrtatm.f

This can be fixed by using the legacy flag in ClimaMake by adding the -std=legacy flag to FCFLAG