configure.in: get rid of clang fatal error

* configure.in (ac_cv_func___builtin_setjmp): get rid of a bug in
  clang 3.3 and 3.4, which __builtin_setjmp() causes fatal error
  in backend, linker error, or segmentation fault.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45515 b2dd03c8-39d4-4d8f-98ff-823fe69b080e