Windows MSVC port

Extend the windows port so it compiles with the toolchain from Visual Studio 2013
diff --git a/py/runtime.c b/py/runtime.c
index a5a5bc5..7a701fe 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -27,6 +27,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <assert.h>
+#include <alloca.h>
 
 #include "mpconfig.h"
 #include "nlr.h"
@@ -1074,11 +1075,12 @@
     uint pkg_name_len;
     const char *pkg_name = mp_obj_str_get_data(dest[0], &pkg_name_len);
 
-    char dot_name[pkg_name_len + 1 + qstr_len(name)];
+    const uint dot_name_len = pkg_name_len + 1 + qstr_len(name);
+    char *dot_name = alloca(dot_name_len);
     memcpy(dot_name, pkg_name, pkg_name_len);
     dot_name[pkg_name_len] = '.';
     memcpy(dot_name + pkg_name_len + 1, qstr_str(name), qstr_len(name));
-    qstr dot_name_q = qstr_from_strn(dot_name, sizeof(dot_name));
+    qstr dot_name_q = qstr_from_strn(dot_name, dot_name_len);
 
     mp_obj_t args[5];
     args[0] = MP_OBJ_NEW_QSTR(dot_name_q);