Add Flang as fortran compiler for mpich-clang

[rfj82982]

I am trying to have flang, which is now part of macport, as fortran compiler for mpich instead of gcc. I try few configure options as described in this page ​https://trac.macports.org/wiki/CompilerSelection, however I always end up to have gcc as fortran compiler. What is the full syntax to force flang instead of gcc? Is this option available. Would be good if among the fortran option for variants flang would be available and would make more sense to have flang as fortran compiler instead of gcc. Same issue seems also to be affecting the openmpi-clang port