msvc: Add a va_copy compatability macro for msvc

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

cmdutils.c

@@ -29,6 +29,7 @@
    references to libraries that are not being built. */
 
 #include "config.h"
+#include "compat/va_copy.h"
 #include "libavformat/avformat.h"
 #include "libavfilter/avfilter.h"
 #include "libavdevice/avdevice.h"

compat/msvcrt/snprintf.c

@@ -24,6 +24,7 @@
 #include <limits.h>
 #include <string.h>
 
+#include "compat/va_copy.h"
 #include "libavutil/error.h"
 
 int avpriv_snprintf(char *s, size_t n, const char *fmt, ...)

compat/va_copy.h

+/*
+ * MSVC Compatible va_copy macro
+ * Copyright (c) 2012 Derek Buitenhuis
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <stdarg.h>
+
+#if !defined(va_copy) && defined(_MSC_VER)
+#define va_copy(dst, src) ((dst) = (src))
+#endif