fix problem that if no copt specified, csrc not works

#### tcc did not refer correct header file
- because test was wrong, standard header files under /usr/include is used.

#### ffi.csrc without configuring with ffi.copt fails
- this simple example produces error.
- also install.sh does not report error when fails.
``` lua
local ffi = require 'ffiex'
local lib,ext = ffi.csrc('test', [[
#include <stdio.h>
extern void test(int id) { printf("%d", id); }
]])
lib.test(1000)
```