[ng] texi: be sure to only emit generic pattern rules once

[ng] texi: be sure to only emit generic pattern rules once

* lib/am/texibuild.am: Only hold generic pattern rules now, the
stuff specific to single '.texi' files moved out ...
* lib/am/texi-spec.am: ... to this new file.
* Makefile.am (dist_am_DATA): Add it.
* automake.in (output_texinfo_build_rules): Process 'texi-spec.am'
rather than 'texbuild.am', and adjust transforms accordingly.
(handle_texinfo): Process 'texibuild.am', once for each valid Texinfo
extension (.texi, .txi, .texinfo).
* t/txinfo-rules-once.sh: New test.

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