dir.c: glob cases on case-insensitive system

* dir.c (glob_make_pattern): all alphabets are magic characters on
  case-insensitive filesystems.  [ruby-core:42469] [Feature #5994]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44802 b2dd03c8-39d4-4d8f-98ff-823fe69b080e