Detect the proper ARCHFLAGS on macOS (#923)
Detect the proper ARCHFLAGS on macOS (#923)

When run `ruby extconf.rb` on macOS,  it did not specify an appropriate file with `file` command.

it just shown the help message.

```
Usage: file [bcCdEhikLlNnprsvzZ0] [-e test] [-f namefile] [-F separator] [-m magicfiles] [-M magicfiles] file...
       file -C -m magicfiles
Try `file --help' for more information.
```
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick