extmk.rb: non-installed extensions cannot link statically

* ext/extmk.rb: exclude extension libraries not to be installed
  unless shared library is built, as they cannot be linked
  statically.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46035 b2dd03c8-39d4-4d8f-98ff-823fe69b080e