common.mk: separate clean-ext
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
common.mk: separate clean-ext
timestamps, and clean-up of subdirectories under ext and gems.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e