build: add new TARNAME variable

That expands to `@PACKAGE_TARNAME@`, similar to the existing
PACKAGE_TARNAME variable.

To make it easier to use (and read) and to be more consistent with the
surrounding variables (NAME and VERSION).

Note that the original PACKAGE_TARNAME is still needed, as by default
(on autoconf v2.69) `docdir=@docdir@` in config.mk.in expands to the
following in config.mk:

    docdir=${datarootdir}/doc/${PACKAGE_TARNAME}

config.mk.in

@@ -8,7 +8,8 @@
 # first target encountered).
 
 NAME=@PACKAGE_NAME@
-PACKAGE_TARNAME=@PACKAGE_TARNAME@
+TARNAME=@PACKAGE_TARNAME@
+PACKAGE_TARNAME=@PACKAGE_TARNAME@ # needed by docdir
 VERSION=@PACKAGE_VERSION@
 
 prefix=@prefix@

config.sh.in

@@ -4,4 +4,5 @@
 # shellcheck disable=SC2034
 
 NAME="@PACKAGE_NAME@"
+TARNAME="@PACKAGE_TARNAME@"
 VERSION="@PACKAGE_VERSION@"