configure.in: expand libdir_basename

* configure.in (libdir_basename): expand with multiarch in configure,
  not to defer the expansion till ruby.pc.in and mkmf.rb.  [Bug #7874]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39347 b2dd03c8-39d4-4d8f-98ff-823fe69b080e