extmk.rb: expand target_prefix
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56065 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extmk.rb: expand target_prefix
timestamp file name.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56065 b2dd03c8-39d4-4d8f-98ff-823fe69b080e