exts.mk.tmpl: fix multiple rubies
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58711 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
exts.mk.tmpl: fix multiple rubies
simultaneously. it can cause making libruby in parallel.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58711 b2dd03c8-39d4-4d8f-98ff-823fe69b080e