configure.in: libdir_basename for -rpath and -install_name

* configure.in (libdir_basename): also -rpath and -install_name flags
  are affected when libruby directory changes.  [Bug #7874]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39345 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-02-21

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39346 b2dd03c8-39d4-4d8f-98ff-823fe69b080e