Since #846, `HAVE_UNWIND_H` is not really in use. Instead we should use `HAVE__UNWIND_BACKTRACE` and `HAVE__UNWIND_GETIP` (added in #846). To prevent that from happening again, also added Bazel tests that confirm stacktrace are still working.
This means that gflags no longer leaks config.h, so I had to fix a bug
in glog.bzl where config.h is generated at the wrong path.
I also switched to the best-practice for depending on git repositories,
ie using http_archive with a mirror.
