Fix test to determine support for backtrace
Fix test to determine support for backtrace

FreeBSD have the required header files and functions we're
using, but it require linking with execinfo to find bactrace(),
and dladdr is part of libc instead of libdl.

Change-Id: I538e3db7ed25a2fd6b9a5048e65b4bed20290cea
Reviewed-on: http://review.couchbase.org/50910
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

vmx/platform,vmx/platform