Upgrading to current makeinfo

[Werner LEMBERG]

To have both `!' and `\!' (and similar `\foo' command and `foo'
property entries) in the index of the info and PDF version of the NR
it is necessary to tag corresponding entries with `@sortas'
directives[*]

  https://sourceforge.net/p/testlilyissues/issues/5551/
  https://sourceforge.net/p/testlilyissues/issues/5552/

`@sortas' was introduced in texinfo 6.1 (released in 2016); I would
thus like to upgrade the minimum version of texinfo to version 6.1
accordingly.  Note that we already use version 6.1 in gub.

Objections?


    Werner


[*] Currently, HTML output is handled by an ancient version of
    `texi2html' that doesn't understand `@sortas' at all; I'm going to
    add a dummy macro replacement to fix that.  As a consequence, the
    index of the HTML output will stay inferior to both the info and
    PDF versions until we upgrade to a recent version of `texi2any'.
    Unfortunately, this is quite a difficult task...