Define FLEXIBLE_ARRAY_MEMBER for MSVC.

src/include/pg_config.h.win32

@@ -46,6 +46,15 @@
 /* Define to the default TCP port number as a string constant. */
 #define DEF_PGPORT_STR "5432"
 
+/* Define to nothing if C supports flexible array members, and to 1 if it does
+   not. That way, with a declaration like `struct s { int n; double
+   d[FLEXIBLE_ARRAY_MEMBER]; };', the struct hack can be used with pre-C99
+   compilers. When computing the size of such an object, don't use 'sizeof
+   (struct s)' as it overestimates the size. Use 'offsetof (struct s, d)'
+   instead. Don't use 'offsetof (struct s, d[0])', as this doesn't work with
+   MSVC and with C++ compilers. */
+#define FLEXIBLE_ARRAY_MEMBER 1
+
 /* Define to 1 if you want National Language Support. (--enable-nls) */
 /* #undef ENABLE_NLS */