Fix _FORTIFY_SOURCE build error

Newer glibc warns if _FORTIFY_SOURCE && __OPTIMIZE__ <= 0.

We are still using no optimize for debugging. So, just commented out
_FORTIFY_SOURCE, instead of something tricky #ifdef. And we can
revisit this and rethink when optimization was default.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>