* vm_dump.c (backtrace): on darwin use custom backtrace() to trace
  beyond _sigtramp. darwin's backtrace can't trace beyond signal
  trampoline with sigaltstack.

* configure.in: check execinfo.h on darwin.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39808 b2dd03c8-39d4-4d8f-98ff-823fe69b080e