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

ext/extmk.rb

@@ -257,6 +257,8 @@ def extmake(target)
 	f.print(*dummy_makefile(CONFIG["srcdir"]))
       end
 
+      return true if !error and target.start_with?("-")
+
       mess = "Failed to configure #{target}. It will not be installed.\n"
       if error
         mess = "#{error}\n#{mess}"