* configure.in: check whether backtrace(3) works well or not.

* vm_dump.c: set HAVE_BACKTRACE 0 if BROKEN_BACKTRACE.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39301 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* vm_dump: FreeBSD ports' libexecinfo's backtrace(3) can't trace
  beyond signal trampoline, and as described in r38342 it can't
  trace on -O because it see stack frame pointers.
  libunwind unw_backtrace see dwarf information in the binary
  and it works with -O (without frame pointers).

* configure.in: remove r38342's hack and check libunwind.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39302 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-02-19

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39303 b2dd03c8-39d4-4d8f-98ff-823fe69b080e