FreeBSD brings it's _Unwind_Backtrace() in their own libunwind,
which is already handled correctly - but Boost does not know
about it and #error's out if _GNU_SOURCE is not defined as it
would require it on Linux.
Latch onto the logic for Apple operating systems, which have
maybe somewhat inherited the conditon from BSD and define
BOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED to make Boost happy.
Christoph Moench-Tegeder