unbreak make dist
issue-605: avoid compilation errors if pthread_key_t is pointer

Which seems to be the case on later cygwin
issue-604: implement runtime-selectable stacktrace capturing

We're now building all supported stacktrace capturing methods. And
there's now a way to select at runtime which method is used.
issue-525: warn user on lack of libunwind but don't fail

Because we now have access to other backtrace capturing method(s) at
runtime.