Use correct headers/libraries when providing xattr support

See https://savannah.gnu.org/patch/index.php?8252. Patch provided
by Anthony G. Basile.

* acinclude.m4 (TAR_HEADERS_ATTR_XATTR_H): Look for <sys/xattr.h>
first and then for <attr/xattr.h>.  Link against libattr.so if
needed.
* lib/xattr-at.h: Include sys/xattr.h or attr/xattr.h, depending
on which one is detected.
* src/Makefile.am [TAR_LIB_ATTR] (tar_LDADD): Link against -lattr.