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/backend/utils/adt/pg_locale.c

@@ -826,6 +826,7 @@ cache_locale_time(void)
 	char	   *bufptr;
 	time_t		timenow;
 	struct tm  *timeinfo;
+	struct tm	timeinfobuf;
 	bool		strftimefail = false;
 	int			encoding;
 	int			i;
@@ -876,7 +877,7 @@ cache_locale_time(void)
 
 	/* We use times close to current time as data for strftime(). */
 	timenow = time(NULL);
-	timeinfo = localtime(&timenow);
+	timeinfo = gmtime_r(&timenow, &timeinfobuf);
 
 	/* Store the strftime results in MAX_L10N_DATA-sized portions of buf[] */
 	bufptr = buf;