I missed something was still using the shell `which`! Tiger's `which` is kind of useless so I wrote a new `which` helper which replaces it:

```ruby
elsif which 'gfortran'
  ohai "Using Tigerbrew-provided fortran compiler."
  puts "This may be changed by setting the FC environment variable."
  self['FC'] = which 'gfortran'
  ...
```

Actually, this could just as well get changed in Homebrew now that I think of it.