Ruby

extmk.rb: silently skip never-installed extensions

• ext/extmk.rb (extmake): as extension libraries start with '-'
  are not installed always, no problems if no errors occurred.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e