Fix compiler warnings

* Add missing function declarations

  int fuse_fs_setattr_x(struct fuse_fs *fs, const char *path,
                        struct setattr_x *attr);
  int fuse_fs_fsetattr_x(struct fuse_fs *fs, const char *path,
                         struct setattr_x *attr, struct fuse_file_info *fi);
  int fuse_reply_xtimes(fuse_req_t req, const struct timespec *bkuptime,
                        const struct timespec *crtime);

* Add missing stdlib.h include
Move headers to /usr/include
Minior code cleanup
Remove Darwin configure script