bug#9822: AMTAR not used

[Karl Berry]

    akim> TAR = tar
    > am__tar = $(TAR) --format=ustar -chf - "$$tardir"
    > am__untar = $(TAR) -xf -
    > would be fine!
    
    stefano> I'd prefer to go with this latter option

As far as I can tell, all that was left to do here is to document the
use of the TAR environment variable in Automake to override "tar" for
"make dist". I just pushed a change for that. It seems the code has been
in place since 1.11.3, judging by the NEWS file.

akim> PS/  Actually if AMTAR/TAR could also set the two troublesome envvars,
akim> that would be wonderful :)

The two troublesome envvars being, I guess, the Mac-specific
COPY_EXTENDED_ATTRIBUTES_DISABLE and COPYFILE_DISABLE,
per https://lists.gnu.org/archive/html/automake/2011-03/msg00135.html.
But I don't much want to fiddle around with MacOSX envvars that may or
may not still work, and might easily be changed or removed at any time.

So ... closing. --best, karl.