compat: automake should define $(mkdir_p), for backward compatibility

compat: automake should define $(mkdir_p), for backward compatibility

That has been unwittingly broken by commit 'v1.12-19-g7a1eb9f'
of 2012-04-28, "AM_PROG_MKDIR_P: deprecate, to be removed in
Automake 1.13".

Report from Benoit Sigoure and Diego Elio Pattenò:
<http://lists.gnu.org/archive/html/automake/2012-07/msg00001.html>

* lib/am/header-vars.am (mkdir_p): Define as an alias for $(MKDIR_P).
* t/list-of-tests.mk (XFAIL_TESTS): Remove 't/mkdir_p.sh'.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>