TAR variable no effect when tar-pax is used

[Jan Engelhardt]

(With automake 1.16.5), your typical Makefile.am produces a Makefile 
like so:

-8<- Makefile
AMTAR = $${TAR-tar}
am__tar = $${TAR-tar} chof - "$$tardir"
am__untar = $${TAR-tar} xf -

dist-gzip: distdir
        tardir=$(distdir) && $(am__tar) | eval GZIP= gzip $(GZIP_ENV) -c 
>$(distdir).tar.gz
        $(am__post_remove_distdir)
->8-

But when you set `tar_pax` in AM_INIT_AUTOMAKE, this comes out:

-8<- Makefile
AMTAR = $${TAR-tar}
am__tar = tar --format=posix -chf - "$$tardir"
am__untar = tar -xf -
->8-

This means one can no longer use `make TAR=/usr/local/bin/tar` to point to e.g. 
a
new development version of tar I've got.

Also, ${AMTAR} seems totally unused. Why is it even there?