configure.in: do not use a variable for longjmp

* configure.in (ac_cv_func___builtin_setjmp): gcc 4.9 disallows a
  variable as the second argument of __builtin_longjmp().
  [ruby-core:61800] [Bug #9692]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45503 b2dd03c8-39d4-4d8f-98ff-823fe69b080e