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