Provide strnlen for MSVC before version 8

dulwich/_objects.c

@@ -29,7 +29,7 @@
 typedef int Py_ssize_t;
 #endif
 
-#if defined(__MINGW32_VERSION) || (defined(__APPLE__) && __MAC_OS_X_VERSION_MIN_REQUIRED < 1070)
+#if defined(__MINGW32_VERSION) || (defined(__APPLE__) && __MAC_OS_X_VERSION_MIN_REQUIRED < 1070) || (defined(_MSC_VER) && _MSC_VER < 1400)
 size_t strnlen(char *text, size_t maxlen)
 {
 	const char *last = memchr(text, '\0', maxlen);