configure-ext.mk.tmpl: fix ext build
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57433 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure-ext.mk.tmpl: fix ext build
and directory, and chomp the last slash.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57433 b2dd03c8-39d4-4d8f-98ff-823fe69b080e