fileutils.rb: split fu_windows?

* lib/fileutils.rb (FileUtils::StreamUtils_#fu_windows?): select
  the method definition for each platforms, and dropped supports
  for deprecated platforms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63465 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: nobu

lib/fileutils.rb

@@ -1145,12 +1145,11 @@ def touch(list, noop: nil, verbose: nil, mtime: nil, nocreate: nil)
   module StreamUtils_
     private
 
-    def fu_windows?
-      if defined?(RbConfig)
-        /mswin|mingw|bccwin|emx/ =~ RbConfig::CONFIG['host_os']
-      else
-        /mswin|mingw|bccwin|emx/ =~ RUBY_PLATFORM
-      end
+    case (defined?(::RbConfig) ? ::RbConfig::CONFIG['host_os'] : ::RUBY_PLATFORM)
+    when /mswin|mingw/
+      def fu_windows?; true end
+    else
+      def fu_windows?; false end
     end
 
     def fu_copy_stream0(src, dest, blksize = nil)   #:nodoc: