merge revision(s) r45716: [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_1@46215 b2dd03c8-39d4-4d8f-98ff-823fe69b080e