configure.in: fix for clang 5.1 __builtin_longjmp

* configure.in (ac_cv_func___builtin_setjmp): __builtin_longjmp() in
  clang 5.1 uses `void**`, not `jmp_buf`.  [Bug #9692]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45508 b2dd03c8-39d4-4d8f-98ff-823fe69b080e