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.

lib/xattr-at.h

@@ -20,7 +20,15 @@
 #define XATTRS_AT_H
 
 #include <sys/types.h>
-#include <attr/xattr.h>
+#if defined(HAVE_SYS_XATTR_H)
+# include <sys/xattr.h>
+#elif defined(HAVE_ATTR_XATTR_H)
+# include <attr/xattr.h>
+#endif
+
+#ifndef ENOATTR
+# define ENOATTR ENODATA        /* No such attribute */
+#endif
 
 /* These are the dir-fd-relative variants of the functions without the
    "at" suffix.  For example, setxattrat (AT_FDCWD, path, name, value, size,