Set FLEXIBLE_ARRAY_MEMBER to empty for MSVC.

Per gripe from Tom Lane. I have tested this with VC 2008, and assume
it will work with earlier versions.

src/include/pg_config.h.win32

@@ -53,7 +53,7 @@
    (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 FLEXIBLE_ARRAY_MEMBER
 
 /* Define to 1 if you want National Language Support. (--enable-nls) */
 /* #undef ENABLE_NLS */