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

CMakeLists.txt

@@ -645,8 +645,10 @@ endif()
 
 if(NOT WIN32)
   find_package(Threads)
-  set(HAVE_THREADS_POSIX ${CMAKE_USE_PTHREADS_INIT})
-  list(APPEND CURL_NETWORK_AND_TIME_LIBS ${CMAKE_THREAD_LIBS_INIT})
+  if(CMAKE_USE_PTHREADS_INIT)
+    set(HAVE_THREADS_POSIX 1)
+    list(APPEND CURL_NETWORK_AND_TIME_LIBS Threads::Threads)
+  endif()
 endif()
 
 if(ENABLE_THREADED_RESOLVER)
@@ -2342,6 +2344,7 @@ if(NOT CURL_DISABLE_INSTALL)
   #   HAVE_GSSAPI
   #   HAVE_LIBIDN2
   #   HAVE_LIBZ ZLIB_VERSION_MAJOR
+  #   HAVE_THREADS_POSIX
   #   HAVE_ZSTD
   #   USE_ARES
   #   USE_BACKTRACE