obsolete: better error message if AM_PROG_MKDIR_P is used

obsolete: better error message if AM_PROG_MKDIR_P is used

In Automake 1.14, the long-deprecated macro AM_PROG_MKDIR_P will be
removed.  Instead of simply dropping the macro definition, which would
cause unclear error messages in aclocal and autoconf:

    configure.ac:4: warning: macro 'AM_PROG_MKDIR_P' not found in library
    configure.ac:4: error: possibly undefined macro: AM_PROG_MKDIR_P
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

we re-add an AM_PROG_MKDIR_P definition that simply raises a *clear* error
message when the macro is used.

See also recent similar commits 'v1.13-5-g9c07d89' and 'v1.13-4-g8e921bf'

* t/am-prog-mkdir-p-no-more.sh: New test.
* t/list-of-tests.mk: Add it.
* m4/obsolete-err.m4: Add the new "error-raising" definition for
AM_PROG_MKDIR_P.

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