init.m4: add probe to check "rm -f" without args work

init.m4: add probe to check "rm -f" without args work

See automake bug#10828.

POSIX will say in a future version that running "rm -f" with no argument
is OK: <http://austingroupbugs.net/view.php?id=542>).

We want to be able to make that assumption in our Makefile recipes.
So we introduce an aggressive probe to check that the usage we want is
actually supported "in the wild" to an acceptable degree.

* m4/init.m4 (AM_INIT_AUTOMAKE): Implement the probe.  To make any issue
more visible, cause the running configure to be aborted by default if
the 'rm' program in use doesn't match our expectations; the user can
still override this though, by setting the ACCEPT_INFERIOR_RM_PROGRAM
environment variable to "yes".

* t/spy-rm.tap: Update heading comments.
* t/rm-f-probe.sh: New test.
* t/list-of-tests.mk: Add it.
* PLANS/rm-f-without-args.txt: Adjust.

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