On all Windows platforms, not just Cygwin, use _timezone and _tzname.

tglsfdc · adunstan · commit 520711d6e23e · 2018-09-12T12:25:57.000-04:00
Back-patch commit 868628e into the 9.5 branch, so that we can support building that branch with Visual Studio 2015. This patch itself could go further back, but other VS2015 patches such as 0fb54de and c8e81af were only back-patched to 9.5, so there seems little point in handling this one differently. Discussion: https://postgr.es/m/CAD=LzWFg+Z-KUS3Wm8-1J2vOuYErJXbjuE6b7quzswQEBXJWMQ@mail.gmail.com Now that we have backported VS2015 support to 9.4 and 9.3, backport this also.
diff --git a/src/include/port.h b/src/include/port.h
@@ -255,12 +255,12 @@ extern int	pgkill(int pid, int sig);
 extern int	pclose_check(FILE *stream);
 
 /* Global variable holding time zone information. */
-#ifndef __CYGWIN__
-#define TIMEZONE_GLOBAL timezone
-#define TZNAME_GLOBAL tzname
-#else
+#if defined(WIN32) || defined(__CYGWIN__)
 #define TIMEZONE_GLOBAL _timezone
 #define TZNAME_GLOBAL _tzname
+#else
+#define TIMEZONE_GLOBAL timezone
+#define TZNAME_GLOBAL tzname
 #endif
 
 #if defined(WIN32) || defined(__CYGWIN__)
