prefer https over http

[Bruno Haible]

Hi,

The internet is moving from http:// URLs to https:// for a while already.
Therefore in HTML documentation generated from texinfo sources, I'd like to
have https:// URLs instead of http:// URLs where possible.

In gnulib I have this piece of texinfo input:
  @ref{Options,,Changing Automake's Behavior,automake,GNU Automake}

It produces a HTML hyperlink to
  http://www.gnu.org/software/automake/manual/automake.html#Options
I've like that it produces a hyperlink to
  https://www.gnu.org/software/automake/manual/automake.html#Options

How to reproduce:
  $ git clone https://git.savannah.gnu.org/git/gnulib.git
  $ cd gnulib/doc; make html
  $ firefox `pwd`/gnulib.html
  $ grep /automake/ gnulib.html

The essential command executed by "make html" is:
  env LANG= LC_MESSAGES= LC_ALL= LANGUAGE= makeinfo --no-split 
--reference-limit=2000 --html -o gnulib.html gnulib.texi

makeinfo is from TeXinfo 6.3, built from source, in my case.

I can't offer a patch: I couldn't find where this URL comes from.

Bruno