bug#13514: unintentional regression with AC_CONFIG_MACRO_DIR in 1.13.1

[Pavel Raiskup]

> [..skip..]
> Maybe the current error should be demoted to a warning, to allow us to
> be more forward-compatible with such possible future scenarios?

I vote for this approach.  It was also proposed some time ago (it is
transitively hyper-linked from here already):

  http://lists.gnu.org/archive/html/automake-patches/2010-02/msg00030.html

It must be adopted to current revision & I would maybe switch the msg channel
from 'unsupported' to 'portability' if I understand its purposes.

Pavel