configure.in: adjust libdirname

* configure.in (libdirname): adjust library path name which libruby
  files will be installed.  [Bug #7874]
* tool/rbinstall.rb (libdir): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39333 b2dd03c8-39d4-4d8f-98ff-823fe69b080e