extmk.rb: don't duplicate

* ext/extmk.rb (extract_makefile): do not add dldflags if duplicated.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40902 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-05-24

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40903 b2dd03c8-39d4-4d8f-98ff-823fe69b080e