atm_gray does not compile on macOS with gfortran

[gaelforget]

This is the error message I get:

Update AD_CONFIG.h and make ./mitgcmuv
make -f Makefile ./mitgcmuv
gfortran -fdefault-real-8 -fdefault-double-8 -fconvert=big-endian -Wunused -Wuninitialized -O3 -ftree-vectorize -funroll-loops  -c atm_phys_driver.for
atm_phys_driver.for:502:10:

  502 |       use radiation_mod
      |          1
Fatal Error: Cannot open module file 'radiation_mod.mod' for reading at (1): No such file or directory
compilation terminated.
make[1]: *** [atm_phys_driver.o] Error 1

when following the README directions using ../../../tools/genmake2 -mods ../code -of ../../../tools/build_options/darwin_amd64_gfortran on MacOS 11.3 and with gfortran v11