netcdf-fortran@4.5: will error if netcdf-c has been built with MPI
support:
```
configure: error:
-----------------------------------------------------------------------
The NetCDF C library is built with parallel I/O feature enabled, but
the Fortran compiler '.../lib/spack/env/gcc/gfortran' supplied in this configure command
does not support MPI-IO. Please use one that does. If parallel I/O
feature is not desired, please use a NetCDF C library with parallel
I/O feature disabled. Abort.
-----------------------------------------------------------------------
```
Copy logic from netcdf-c to add an `mpi` variant.
52eba8fb00