bug#7657: TEXINFOS primary accepts too many prefixes.

[Stefano Lattarini]

Currently, the TEXINFOS primary accept *all* the standard automake
builtin prefix dirs (from `lib' and `bin' to `pkglibexec' and
`sysconf').

IMHO it should accept only the `info' prefix (maybe also `doc'? but
I'd rather say no).

Thanks,
   Stefano