configure.in: fix rpath flag

* configure.in (RPATHFLAG): use -rpath if -R is not available.

* configure.in (LIBRUBY_RPATHFLAGS): use RPATHFLAG instead of
  hardcoded -R option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44506 b2dd03c8-39d4-4d8f-98ff-823fe69b080e