Fix for lack of va_copy() on certain Windows versions

Based-on-patch-by: David Rowley <dgrowleyml@gmail.com>

src/include/port/win32.h

@@ -237,6 +237,15 @@ int			setitimer(int which, const struct itimerval * value, struct itimerval * ov
 #endif
 #endif
 
+/*
+ * Supplement to <stdarg.h>
+ */
+
+/* Visual Studios 2012 and earlier don't have va_copy() */
+#if defined(_MSC_VER) && _MSC_VER <= 1700
+#define va_copy(dest, src) ((dest) = (src))
+#endif
+
 /*
  * Supplement to <sys/types.h>.
  *