Don't #define snprintf in VS 2015 or higher.

In VS 2015 and higher snprintf is supplied and therefore vsnprintf
doesn't need to be defined. This also avoids problems caused by
_snprintf being different from snprintf.

This fixes a build break with VS 2015 and improves security.

Originally submitted via chromium by brucedawson@chromium.org
https://codereview.chromium.org/1055603003

Additionally break this MSVC-specific tweak to a new file, which will
become the home of all such MSVC-specific things.

This requires adding a dependency on msvc.h to every example which uses
args.c and tools_common.h

Change-Id: I35b5f8e7ea00f6627403aabc9ea79b0412557a99

args.c

@@ -14,9 +14,7 @@
 #include <limits.h>
 #include "args.h"
 
-#ifdef _MSC_VER
-#define snprintf _snprintf
-#endif
+#include "vpx_ports/msvc.h"
 
 #if defined(__GNUC__) && __GNUC__
 extern void die(const char *fmt, ...) __attribute__((noreturn));