Merge #18358: util: fix compilation with mingw-w64 7.0.0

a46484c8b3715725f5dc0b8ad1bf921880ed9af1 build: Detect gmtime_* definitions via configure (Ben Woosley)

Pull request description:

  Something has changed in the mingw-w64 headers such that we
  no-longer compile when using 7.0.0.

  ```bash
  util/time.cpp: In function 'std::__cxx11::string FormatISO8601DateTime(int64_t)':
  util/time.cpp:84:9: error: 'gmtime_r' was not declared in this scope
       if (gmtime_r(&time_val, &ts) == nullptr) {
           ^~~~~~~~
  util/time.cpp: In function 'std::__cxx11::string FormatISO8601Date(int64_t)':
  util/time.cpp:97:9: error: 'gmtime_r' was not declared in this scope
       if (gmtime_r(&time_val, &ts) == nullptr) {
  ```

  Looking at time.h, it seems that `gmtime_r()` is only available when
  `_POSIX_C_SOURCE` is defined. This must have been the case for 6.0.0
  (which we compile fine using), but no-longer seems to be for 7.0.0?

  I've checked that adding `-D_POSIX_C_SOURCE=200112L` to our compile
  flags does fix the issue above.

  However, an alternative solution seems to be to just use `gmtime_s()`
  instead, when compiling with `mingw-w64`, as `gmtime_r()` [just wraps
  `gmtime_s()` anyways](https://github.com/mirror/mingw-w64/blob/7c03b11bf173944679102bbe0ac061842e2f594b/mingw-w64-headers/crt/time.h#L284).

  I've tested this change crosss-compiling on Debian Bullseye ([mingw-w64 7.0.0](https://packages.debian.org/source/bullseye/mingw-w64))
  and Buster ([mingw-w64 6.0.0](https://packages.debian.org/source/buster/mingw-w64)).

ACKs for top commit:
  laanwj:
    ACK a46484c8b3715725f5dc0b8ad1bf921880ed9af1

Tree-SHA512: 7cf1a81060b9625d64de40b77341d74704cc8ae1358d25d7e2909685dc83a7a9762260d72e47806e9f0a5cbabf88d0239ec9e0fd0ebd3731b1d206b075f43a63

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