in nf-config, --has-f90 flag is blank with autoconf builds

[Edward Hartnett]

Note that the --has-f90 flag is blank:

/usr/local/netcdf-fortran/bin/nf-config --all

This netCDF-Fortran 4.6.0-development has been built with the following features: 

  --cc        -> gcc
  --cflags    -> -I/usr/local/netcdf-fortran/include -I/usr/local/netcdf-c/include

  --fc        -> gfortran
  --fflags    -> -I/usr/local/netcdf-fortran/include -I/usr/local/netcdf-fortran/include
  --flibs     -> -L/usr/local/netcdf-fortran/lib -lnetcdff -L/usr/local/netcdf-c/lib -lnetcdf -lnetcdf -ldl -lm 
  --has-f90   -> 
  --has-f03   -> yes

  --has-nc2   -> yes
  --has-nc4   -> yes

  --prefix    -> /usr/local/netcdf-fortran
  --includedir-> /usr/local/netcdf-fortran/include
  --version   -> netCDF-Fortran 4.6.0-development

This was pointed out in a netcdf-c issue: Unidata/netcdf-c#2276