docs: improve description of ${PACKAGE}, ${VERSION}, and similar variables

docs: improve description of ${PACKAGE}, ${VERSION}, and similar variables

In particular, suggesting that $PACKAGE and $VERSION should be aliases of
the $PACKAGE_TARNAME and $PACKAGE_VERSION definitions coming from AC_INIT,
and not be defined via an obsolete 2-argument invocation of AM_INIT_AUTOMAKE;
and why that is the best default, given all our historical baggage.

See discussion in http://debbugs.gnu.org/16623 for more information and
background.

* doc/automake.texi: Adjust.
* THANKS: Update.

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