compat: automake should substitute @mkdir_p@, for backward compatibility

compat: automake should substitute @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". We thought it
wasn't a big deal, but Jim Meyering reported that @mkdir_p@ is used in
gettext's Makefile.in.in template:
<http://lists.gnu.org/archive/html/automake/2012-07/msg00014.html>

* lib/am/header-vars.am (mkdir_p): Don't define.
* m4/init.m4 (AM_INIT_AUTOMAKE): AC_SUBST 'mkdir_p' with $(MKDIR_P).
* t/mkdir_p.sh, t/mkdirp-deprecation.sh: Enhance.
* NEWS: Update.

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