configure.in: use the first candidate

* configure.in (ac_cv_func___builtin_setjmp): use the first cast
  which works with __builtin_longjmp().  [Bug #9692]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45509 b2dd03c8-39d4-4d8f-98ff-823fe69b080e