glibc runs getaddrinfo_a() lookups in a thread it creates internally,
bypassing the pthread_create interceptor, so TSan never initializes that
thread and it crashes on its first allocation. Detect TSan (GCC's
__SANITIZE_THREAD__ / Clang's __has_feature(thread_sanitizer)) and use the
existing std::thread-based fallback instead, which keeps the lookup timeout
working. Non-TSan builds are unchanged.
