mkmf.rb: get rid of error messages
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57727 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: get rid of error messages
when the extension library will be build, to get rid of trying
to remove $(TARGET_SO_DIR).
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57727 b2dd03c8-39d4-4d8f-98ff-823fe69b080e