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}
To note on the output files that they are generated and to clarify how
they are generated.
From the manual of GNU Autoconf (version 2.69):
> -- Variable: configure_input
> A comment saying that the file was generated automatically by
> 'configure' and giving the name of the input file. 'AC_OUTPUT'
> adds a comment line containing this variable to the top of every
> makefile it creates. For other files, you should reference this
> variable in a comment at the top of each input file. For
> example, an input shell script should begin like this:
>
> #!/bin/sh
> # @configure_input@
>
> The presence of that line also reminds people editing the file
> that it needs to be processed by 'configure' in order to be used.
Resulting output on config.mk:
# config.mk. Generated from config.mk.in by configure.
Relates to #5140.
Currently, the configure-time variables (that is, the ones that assign
to placeholders, such as "@HAVE_MAN@", which are set/replaced at
configure-time) are defined on multiple files (such as on Makefile.in
and on common.mk.in).
To avoid duplication, centralize these variables on a single file
(config.mk.in) and replace all of the other definitions of them with an
include of config.mk.
