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  XP likely needs a newer version of this DLL, or may not run.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

lib/config-os400.h

@@ -108,6 +108,9 @@
 /* Define if you have the GNU gssapi libraries */
 #undef HAVE_GSSGNU
 
+/* Define if you have the `localtime_r' function. */
+#define HAVE_LOCALTIME_R
+
 /* Define if you have the <netdb.h> header file. */
 #define HAVE_NETDB_H
 

lib/config-plan9.h

@@ -86,6 +86,7 @@
 #define HAVE_LIBGEN_H 1
 #define HAVE_LIBZ 1
 #define HAVE_LOCALE_H 1
+#define HAVE_LOCALTIME_R 1
 #define HAVE_LONGLONG 1
 #define HAVE_NETDB_H 1
 #define HAVE_NETINET_IN_H 1

lib/curl_config.h.cmake

@@ -391,6 +391,9 @@
 /* Define to 1 if you have the <locale.h> header file. */
 #cmakedefine HAVE_LOCALE_H 1
 
+/* Define to 1 if you have a working localtime_r function. */
+#cmakedefine HAVE_LOCALTIME_R 1
+
 /* Define to 1 if the compiler supports the 'long long' data type. */
 #cmakedefine HAVE_LONGLONG 1
 

lib/curl_setup.h

@@ -94,8 +94,7 @@
 #define _CRT_NONSTDC_NO_DEPRECATE  /* for close(), fileno(), unlink(), etc. */
 #endif
 #ifndef _CRT_SECURE_NO_WARNINGS
-#define _CRT_SECURE_NO_WARNINGS  /* for getenv(), strcpy(),
-                                    in tests: localtime(), sscanf() */
+#define _CRT_SECURE_NO_WARNINGS  /* for getenv(), strcpy(), tests: sscanf() */
 #endif
 #endif /* _MSC_VER */