[ng] texi: verbatim include of rules for texinfo installation

[ng] texi: verbatim include of rules for texinfo installation

It tells whether the '.info' files must be installed by default (usually
yes, unless the "no-installinfo" automake option is used).  This is just
a preparatory change, in view of future ones.

* lib/am/texinfos.am: Move ...
* lib/am/texinfos.mk: ... here, with related adjustments.  Also ...
(info_TEXINFOS): ... initialize this to empty by default.
* lib/am/automake.in (handle_texinfo): Adjust and simplify accordingly.
* Makefile.am (dist_am_DATA): Adjust.
* t/txinfo10.sh: Remove, basically an obsolete grepping check.  We can
instead ...
* t/txinfo21.sh: ... enhance this semantic test to subsume and supersede
the removed one.
* t/override-conditional-1.sh: Adjust to avoid spurious failures.

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