sheepfs: use big_writes option only when the version of libfuse is equal or greater than 2.8.0

When using sheepfs on rhel5 environment, it will report:

    fuse: unknown option `big_writes'

because the 'big_writes' option is only supported by libfuse with version >= 2.8.0.

So we should add check for version of libfuse in configure.ac.

Signed-off-by: RobinDong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>