configure.in: refine __builtin_unreachable check

* configure.in (rb_cv_func___builtin_unreachable): try with an
  external variable not only by a warning, which might not be
  shown due to the optimization.  [ruby-core:61647] [Bug #9665]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45716 b2dd03c8-39d4-4d8f-98ff-823fe69b080e