localtime: detect thread-safe alternatives and use them

- add local API `toolx_localtime()` to wrap the banned function
  `localtime()`. Used from libcurl, libtests and test servers.
- auto-detect and use `localtime_r()` where available (e.g. Linux).
  Also to support multi-threading.
- use `localtime_s()` on Windows. It requires MSVC or mingw-w64 v4+.
  Also to support multi-threading.
  Use local workaround to also support mingw-w64 v3.
- add `src/toolx` to keep internal APIs used by the curl tool and tests,
  but not by libcurl. `toolx_localtime()` is the first API in it.
- replace `localtime()` calls with `toolx_localtime()`.
  Except in examples.
- note Windows XP's default `msvcrt.dll` doesn't offer secure CRT APIs.
  XP likely needs a newer version of this DLL, or may not run.
- note that `localtime()` mirrors `gmtime()`, with the difference that
  `gmtime()`'s internal wrapper lives in curlx.

Also:
- drop redundant `int` casts.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/localtime-localtime32-localtime64
https://learn.microsoft.com/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
https://pubs.opengroup.org/onlinepubs/9799919799/functions/localtime.html
https://linux.die.net/man/3/localtime_r

Ref: #19955 (for `gmtime_r()`)
Follow-up to 54d9f060b4
Closes #19957

CMake/unix-cache.cmake

@@ -169,6 +169,7 @@ else()
   set(HAVE_LINUX_TCP_H 0)
 endif()
 set(HAVE_LOCALE_H 1)
+set(HAVE_LOCALTIME_R 1)
 set(HAVE_LONGLONG 1)
 if(APPLE)
   set(HAVE_MACH_ABSOLUTE_TIME 1)

CMake/win32-cache.cmake

@@ -123,6 +123,7 @@ set(HAVE_IOCTL_SIOCGIFADDR 0)
 set(HAVE_IO_H 1)
 set(HAVE_LINUX_TCP_H 0)
 set(HAVE_LOCALE_H 1)
+set(HAVE_LOCALTIME_R 0)
 set(HAVE_MEMRCHR 0)
 set(HAVE_MSG_NOSIGNAL 0)
 set(HAVE_NETDB_H 0)