Attached is a new full diff:
  * (configure.ac) Try to detect thread-safe resolver and refuse to
allow --enable-threaded-resolve unless thread-safe resolver detected)
  * (hostthre.c) Threaded resolver uses getaddrinfo whenever available
  * (hostip4.c) "Thread safe" IPv4 resolve factored out into
Curl_ipv4_resolve. Used by both synchronous and threaded resolver for
IPv4 resolves.
  * (hostip4.c) Curl_ipv4_resolve uses getaddrinfo if present on the
system (both synchronous and threaded). This will make --disable-ipv6
thread safe on the Mac.
  * Some code simplification - async callbacks, fewer macros, got rid
of hostent_dup/hostent_free/gethostbyname_callback from previous
patch, also got rid of Curl_addrinfo_copy and Curl_addrinfo6_callback.

-Costa

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