cmake: use Threads::Threads imported target for POSIX Threads

- use `Threads::Threads` imported target, replacing
  `CMAKE_THREAD_LIBS_INIT`. To use the modern form and to allow using
  `THREADS_PREFER_PTHREAD_FLAG` option.

- only add Threads library/options if POSIX Threads was detected (not
  any other threading backend FindThreads may detect, e.g. the HP one.)

- curl-config.in.cmake: detect and define `Threads::Threads`.

Refs:
https://cmake.org/cmake/help/v3.18/module/FindThreads.html
https://cmake.org/cmake/help/v4.3/module/FindThreads.html

Closes #21163

CMake/curl-config.in.cmake

@@ -35,6 +35,10 @@ endif()
 
 include(CMakeFindDependencyMacro)
 
+if("@HAVE_THREADS_POSIX@")
+  find_dependency(Threads)  # for Threads::Threads
+endif()
+
 if("@USE_OPENSSL@")
   if("@OPENSSL_VERSION_MAJOR@")
     find_dependency(OpenSSL "@OPENSSL_VERSION_MAJOR@")