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merge revision(s) 45716: [Backport #9665]

    * 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/branches/ruby_2_0_0@46201 b2dd03c8-39d4-4d8f-98ff-823fe69b080e