Add missing file from CMake patch set.

port/snprintf.c

@@ -0,0 +1,37 @@
+/**
+ * Workaround for lack of snprintf(3) in Visual Studio.  See
+ * http://stackoverflow.com/questions/2915672/snprintf-and-visual-studio-2010/8712996#8712996
+ * It's a trivial wrapper around the builtin _vsnprintf_s and
+ * _vscprintf functions.
+ */
+
+#ifdef _MSC_VER
+
+#include <stdio.h>
+#include <stdarg.h>
+
+int vsnprintf(char* str, size_t size, const char* format, va_list ap)
+{
+  int count = -1;
+
+  if (size != 0)
+    count = _vsnprintf_s(str, size, _TRUNCATE, format, ap);
+  if (count == -1)
+    count = _vscprintf(format, ap);
+
+  return count;
+}
+
+int snprintf(char* str, size_t size, const char* format, ...)
+{
+  int count;
+  va_list ap;
+
+  va_start(ap, format);
+  count = vsnprintf(str, size, format, ap);
+  va_end(ap);
+
+  return count;
+}
+
+#endif // _MSC_VER