texi: deprecate hack about info files in CLEANFILES variables

texi: deprecate hack about info files in CLEANFILES variables

For quite a long time, Automake has been implementing an undocumented
hack which ensured that '.info' files which appeared to be cleaned (by
e.g. being listed in the CLEANFILES or DISTCLEANFILES variables) were
built in the builddir rather than in the srcdir; this hack was introduced
to ensure better backward-compatibility with packages such as Texinfo,
which did things like:

    info_TEXINFOS = texinfo.txi info-stnd.texi info.texi
    DISTCLEANFILES = texinfo texinfo-* info*.info*
    # Do not create info files for distribution.
    dist-info:
        @:

in order not to distribute .info files.

Now that we have the 'info-in-builddir' option that explicitly causes
generated '.info' files to be placed in the builddir, this hack should
be longer necessary, so we deprecate it with runtime warnings.  It is
scheduled to be removed altogether in Automake 1.14.

* automake.in (handle_texinfo_helper): Raise proper runtime warnings
if the hack is triggered.
* NEWS: Update.
* t/txinfo28.sh: Adjust.
* t/txinfo23.sh: Likewise.
* t/txinfo25.sh: Adjust and extend.
* t/txinfo24.sh: Likewise.

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