thread-safety: gmtime_r(), localtime_r()

Use gmtime_r() and localtime_r() instead of gmtime() and localtime(),
for thread-safety.

There are a few affected calls in libpq and ecpg's libpgtypes, which
are probably effectively bugs, because those libraries already claim
to be thread-safe.

There is one affected call in the backend.  Most of the backend
otherwise uses the custom functions pg_gmtime() and pg_localtime(),
which are implemented differently.

While we're here, change the call in the backend to gmtime*() instead
of localtime*(), since for that use time zone behavior is irrelevant,
and this side-steps any questions about when time zones are
initialized by localtime_r() vs localtime().

Portability: gmtime_r() and localtime_r() are in POSIX but are not
available on Windows.  Windows has functions gmtime_s() and
localtime_s() that can fulfill the same purpose, so we add some small
wrappers around them.  (Note that these *_s() functions are also
different from the *_s() functions in the bounds-checking extension of
C11.  We are not using those here.)

On MinGW, you can get the POSIX-style *_r() functions by defining
_POSIX_C_SOURCE appropriately before including <time.h>.  This leads
to a conflict at least in plpython because apparently _POSIX_C_SOURCE
gets defined in some header there, and then our replacement
definitions conflict with the system definitions.  To avoid that sort
of thing, we now always define _POSIX_C_SOURCE on MinGW and use the
POSIX-style functions here.

Reviewed-by: Stepan Neretin <sncfmgg@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/eba1dc75-298e-4c46-8869-48ba8aad7d70@eisentraut.org

src/include/port/win32_port.h

@@ -420,6 +420,19 @@ extern int	_pglstat64(const char *name, struct stat *buf);
  */
 #define strtok_r strtok_s
 
+/*
+ * Supplement to <time.h>.
+ */
+#ifdef _MSC_VER
+/*
+ * MinGW has these functions if _POSIX_C_SOURCE is defined.  Third-party
+ * libraries might do that, so to avoid clashes we get ahead of it and define
+ * it ourselves and use the system functions provided by MinGW.
+ */
+#define gmtime_r(clock, result) (gmtime_s(result, clock) ? NULL : (result))
+#define localtime_r(clock, result) (localtime_s(result, clock) ? NULL : (result))
+#endif
+
 /*
  * Locale stuff.
  *