Gauche 0.9.15 fails to build (test failure)

It appears the FFI test is failing (at least for x86_64 target):

```

Testing ffitest ... failed.

9 discrepancies found:

test load-foreign: expects (#t (f_i () i) (f_i_i (i) i)) => got #<<error> ": no matching clause for (.struct _IO_cookie_io_functions_t () ((read (.pointer () (.type cookie_read_function_t () (.function () (.type __ssize_t () (long ())) ((__cookie (.pointer () (void ()))) (__buf (.pointer () (char ()))) (__nbytes (.type size_t () (u-long ())))))))) (write (.pointer () (.type cookie_write_function_t () (.function () (.type __ssize_t () (long ())) ((__cookie (.pointer () (void ()))) (__buf (.pointer () (char (const)))) (__nbytes (.type size_t () (u-long ())))))))) (seek (.pointer () (.type cookie_seek_function_t () (.function () (int ()) ((__cookie (.pointer () (void ()))) (__pos (.pointer () (.type __off64_t () (long ())))) (__w (int ()))))))) (close (.pointer () (.type cookie_close_function_t () (.function () (int ()) ((__cookie (.pointer () (void ()))))))))))">

test call-foreign f_c: expects 9 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_i: expects 42 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_f: expects 1.25 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_d: expects 3.14 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_v: expects #<undef> => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_i_i: expects 99 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_f_f: expects 0.125 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

test call-foreign f_d_d: expects 1.0 => got #<<error> "flib is supposed to be of type #<class <foreign-library>>, but got #f">

```