* configure.in: Use sigsetjmp by default so jumping out of signal
  handlers properly restores the signal mask and SS_ONSTACK flag.
  [ruby-core:54175] [Bug #8254]

* configure.in: Manually check for presence of sigsetjmp. It is not a
  function on some systems, so AC_CHECK_FUNCS cannot be used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40402 b2dd03c8-39d4-4d8f-98ff-823fe69b080e