compat: reinstate AM_PROG_MKDIR_P, for gettext

compat: reinstate AM_PROG_MKDIR_P, for gettext

Do not remove AM_PROG_MKDIR_P just yet.

gettext (latest from git) still AC_REQUIRE's AM_PROG_MKDIR_P via its
intl.m4 and po.m4 files, which are pulled into *many* projects.

When I try to build one of those projects (coreutils) using the latest
from automake.git/master, I see this failure:

    $ aclocal -I m4
    configure.ac:477: warning: AM_PROG_MKDIR_P is m4_require'd \
      but not m4_defun'd
    m4/po.m4:23: AM_PO_SUBDIRS is expanded from...
    m4/gettext.m4:57: AM_GNU_GETTEXT is expanded from...
    configure.ac:477: the top level

That is because AM_PROG_MKDIR_P was removed (via commit
v1.12-20-g8a1c64f) in preparation for the next release of automake.

* NEWS: Remove the paragraph that announced the removal of
AM_PROG_MKDIR_P.
* Makefile.am (dist_automake_ac_DATA): Add m4/mkdirp.m4.
* m4/mkdirp.m4: Re-add file.
* t/mkdirp-deprecation.sh: Likewise.
* t/list-of-tests.mk: Add it.
* automake.in: Restore removed code, and adjust comments, s/1.13/1.14/
to reflect new plan for removal.
* doc/automake.texi (Obsolete Macros): Restore the section, but
now with only one entry: the one for AM_PROG_MKDIR_P.