curlx: move Curl_gmtime(), use gmtime_s() on Windows

Move `Curl_gmtime()` to curlx and rename to `curlx_gmtime()`. Then call
the internal wrapper also from the curl tool, to avoid using the banned
`gmtime()` directly, and using better, thread-safe alternatives when
available.

Windows `gmtime_s()` requires mingw-w64 v4+ or MSVC. Use local
workaround to also support mingw-w64 v3. `gmtime_s()` also makes
defining `_CRT_SECURE_NO_WARNINGS` unnecessary.

Also:
- lib: drop unused `parsedate.h` includes.
- drop redundant cast from `gmtime_r()` result.
- autotools: reverse condition in the proto detection to avoid
  misleading readers. (the condition plays no role in detection.)
- 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  XP likely needs a newer version of this DLL, or may not run.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/gmtime-gmtime32-gmtime64
https://learn.microsoft.com/cpp/c-runtime-library/reference/gmtime-s-gmtime32-s-gmtime64-s
https://pubs.opengroup.org/onlinepubs/9799919799/functions/gmtime.html
https://linux.die.net/man/3/gmtime_r

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

src/tool_writeout.c

@@ -548,7 +548,6 @@ static const char *outtime(const char *ptr, /* %time{ ... */
   ptr += 6;
   end = strchr(ptr, '}');
   if(end) {
-    struct tm *utc;
     struct dynbuf format;
     char output[256]; /* max output time length */
 #ifdef HAVE_GETTIMEOFDAY
@@ -602,10 +601,10 @@ static const char *outtime(const char *ptr, /* %time{ ... */
         result = curlx_dyn_addn(&format, &ptr[i], 1);
     }
     if(!result) {
-      /* !checksrc! disable BANNEDFUNC 1 */
-      utc = gmtime(&secs);
-      if(curlx_dyn_len(&format) && utc &&
-         strftime(output, sizeof(output), curlx_dyn_ptr(&format), utc))
+      struct tm utc;
+      result = curlx_gmtime(secs, &utc);
+      if(curlx_dyn_len(&format) && !result &&
+         strftime(output, sizeof(output), curlx_dyn_ptr(&format), &utc))
         fputs(output, stream);
       curlx_dyn_free(&format);
     }