Avoid VPATH-related distcheck failures with non-GNU make

[Ralf Wildenhues]

OK, so all the added '$MAKE distcheck' "for safety" at the end of the
tests caused roughly a thousand new test failures across the board, it
seems, drowning other signals.  Most of these failures apparently have
two reasons:

1) Cleanup failed to remove some directory because of stale NFS handles,

2) The native make is not up to the task of VPATH builds.

Now, distcheck is not a GCS-mandated target, so it need not work
everywhere.  Also, we already document in the GNU-global INSTALL file
that GNU make is preferred when doing VPATH builds (although Automake
currently still does try to work around all issues except those from
Solaris 2.6 and IRIX make for the standard targets).

So, how about changing *those* $MAKE distcheck invocations which don't
have added value over "checking that the distribution is complete" to
call ensure_distcheck_, with this in defs.in:

ensure_distcheck_ ()
{
  if $MAKE --version -v | grep GNU; then
    $MAKE distcheck
  else
    :
  fi
}

(not turning the test result into a SKIP since this is not a major point
of the test).

Remains only the task to identify for which tests this is true.


Addressing (1) right may turn out to be more tricky: if there are files
which are held open by some process, then that would probably be a bug
in the test.  Otherwise, I'm suspecting bugs in the NFS implementation.
Maybe some of the clean rules are lacking '-' prefix (to ignore return
status), but that should only be added for those rules where we really
cannot miss an important failure (e.g., bug in the rule) that we must
address.

Thanks,
Ralf