downloader.rb: true symlink on cygwin

* tool/downloader.rb (Downloader.link_cache): on Cygwin, make true
  symlink only, which is provided by the OS.  as mingw/mswin ruby
  can't follow cygwin's pseudo symlink, it will fail on the same
  source tree.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58843 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/downloader.rb

@@ -225,6 +225,7 @@ class Downloader
   def self.link_cache(cache, file, name, verbose = false)
     return false unless cache and cache.exist?
     return true if cache.eql?(file)
+    if /cygwin/ !~ RUBY_PLATFORM or /winsymlink:nativestrict/ =~ ENV['CYGWIN']
       begin
         file.make_symlink(cache.relative_path_from(file.parent))
       rescue SystemCallError
@@ -235,6 +236,7 @@ class Downloader
         end
         return true
       end
+    end
     begin
       file.make_link(cache)
     rescue SystemCallError