* 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