FileStore: support compiling without libxfs

When configured with --without-libxfs, use GenericFileStoreBackend
instead of XfsFileStoreBackend for XFS.  At this point this would only
impact the allocation hint op.  The default is to compile with
--with-libxfs.  (Previously it was unconditionally enabled on linux and
disabled for non-linux arches.)

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1412 from ceph/wip-libxfs-flag

FileStore: support compiling without libxfs

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>