AM_PROG_MKDIR_P: deprecate, to be removed in Automake 1.13

AM_PROG_MKDIR_P: deprecate, to be removed in Automake 1.13

Support for the obsolescent 'AM_PROG_MKDIR_P' macro (and its
output variable '$(mkdir_p)') has already been deprecated in
the documentation.  It's now time to make its use actively
emit warnings in the 'obsolete' category as well.

* automake.in (scan_autoconf_traces): Warn about 'AM_PROG_MKDIR_P'
obsolescent if that macro is seen.
* m4/mkdirp.m4 (AM_PROG_MKDIR_P): Warn about its own obsolescence.
* m4/init.m4: Require 'AC_PROG_MKDIR_P', not 'AM_PROG_MKDIR_P'.
* t/mkdirp-deprecation.sh: New test.
* t/list-of-tests.mk: Add it.
* syntax-check.mk (sc_mkdir_p): Delete this check as obsolete.
(syntax_check_rules): Remove it.
* t/gettext-macros.sh: Ensure the gettext-requiring tests will
call aclocal and automake with the 'obsolete' warnings disabled,
since even recent versions of gettext used the now-deprecated
'AM_PROG_MKDIR_P' m4 macro.
* NEWS: Update.

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