configure: Allow disabling pthreads, fall back on Win32 threads

[jay]

When the threaded resolver is used pthreads is the default, however this
change makes it possible to --disable-pthreads and then configure can
fall back on Win32 threads for native Windows builds.

Closes #xxxx

---

Context from ./configure --help output:

  --enable-threaded-resolver
                          Enable threaded resolver
  --disable-threaded-resolver
                          Disable threaded resolver
  --enable-pthreads       Enable POSIX threads (default for threaded resolver)
  --disable-pthreads      Disable POSIX threads

Context from curl_config.h when mingw ./configure --disable-pthreads --enable-threaded-resolver:

/* if you want POSIX threaded DNS lookup */
/* #undef USE_THREADS_POSIX */

/* if you want Win32 threaded DNS lookup */
#define USE_THREADS_WIN32 1

[mention-bot]

@jay, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bagder, @yangtse and @dfandrich to be potential reviewers.

[bagder]

Looks fine to me!