Fix build error with Xcode 4 or earlier

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2617 f4f36d40-9719-0410-9868-dde021a23eda
Fix NSInvalidArgumentException in osx_mf_objc_classnames() on OS X 10.8 or earlier

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2618 f4f36d40-9719-0410-9868-dde021a23eda