common.mk: directory timestamps
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55693 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
common.mk: directory timestamps
directories under the specific directory, to get rid of match
with files under the source directory.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55693 b2dd03c8-39d4-4d8f-98ff-823fe69b080e