Summary
-------

POSIX will say in a future version that calling "rm -f" with no argument
is OK; and this sensible behaviour seem to be already very widespread in
"the wild" (and possibly lacking only on those systems that are well on
their way to obsolescence).

Se we'd like to simplify several automake-generated "cleaning" rules
accordingly, to get rid of the awful idiom:

  test -z "$(VAR)" || rm -f $(VAR)

See automake bug#10828.

For Automake 1.14 (DONE)
------------------------

Add a temporary "probe check" in AM_INIT_AUTOMAKE that verifies that
the no-args "rm -f" usage is supported on the system configure is
being run on; complain loudly if this is not the case, and tell the
user to report the situation to us.

For Automake 2.0
----------------

Make any failure in the configure-time probe check introduced by the
previous point fatal; and in case of failure, also suggest to the user
to install an older version of GNU coreutils to work around the
limitation of his system (this version should be old enough not to
be bootstrapped with Automake 2.0, otherwise the user will face a
bootstrapping catch-22).

In all our recipes, start assuming "rm -f" with no argument is OK;
simplify and de-uglify the recipes accordingly.

For Automake 3.0
----------------

Remove the runtime probe altogether.