remove obsolete instructions on hacking clang+gfortran (#2217)
remove obsolete instructions on hacking clang+gfortran (#2217)

* remove obsolete instructions on hacking clang+gfortran

* add detailed instructions on how to setup Clang+gfortran on macOS

LLNL/spack,lgarren/spack,TheTimmy/spack,krafczyk/spack,iulian787/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,EmreAtes/spack,EmreAtes/spack,LLNL/spack,skosukhin/spack,skosukhin/spack,tmerrick1/spack,mfherbst/spack,iulian787/spack,skosukhin/spack,TheTimmy/spack,TheTimmy/spack,EmreAtes/spack,TheTimmy/spack,mfherbst/spack,mfherbst/spack,tmerrick1/spack,lgarren/spack,krafczyk/spack,lgarren/spack,LLNL/spack,tmerrick1/spack,tmerrick1/spack,matthiasdiener/spack,TheTimmy/spack,matthiasdiener/spack,mfherbst/spack,matthiasdiener/spack,lgarren/spack,EmreAtes/spack,krafczyk/spack,EmreAtes/spack,iulian787/spack,lgarren/spack,iulian787/spack,krafczyk/spack,skosukhin/spack,LLNL/spack,krafczyk/spack,skosukhin/spack,matthiasdiener/spack,iulian787/spack,LLNL/spack