I donœôòùt know how to keep the @ifinfo trick

I see no need for @ifinfo.  It has no effect inside @titlepage anyway,
since @titlepage ... @end titlepage is ignored for Info output.

    nearly the same (more on that later) copyright notice
    is repeated three times.

1) @copying should come first.

2) @insertcopying can/should be used to insert the copying text in
@titlepage and anywhere else.  I created @insertcopying at the same time
as @copying, precisely to avoid having to write the copying text more
than once.

3) I see no need for @macro.  I also don't think a YEARS variable makes
sense.  The years will be different for different people.

4) I see no need for a separate EDITION and UPDATED in the
@titlepage text, but whatever.

5) The period outside the quotes in the standard text is intentional
(per fdl.texi).  Not that it's the end of the world to have it inside.

6) All this is shown in the "short example" document that is in the
Texinfo manual.  

Appended is another version of your frontmatter this way.  (Probably
with the accented characters messed up, but you get the idea.)

BTW, rms prefers that the Top node not include @insertcopying, but it's
up to you.  (I see I haven't taken @insertcopying out of the Top node
in the Texinfo manual's example document.  Oops.)

BTW 2, the spaces in your "guix package" and "guix build" dir entries 
make completion painful.  Also you're missing periods after the node
names.  Also the dir file is much more readable if entries are kept on
one line at all costs.  I suggest:
@direntry
* Guix (guix).                      Guix, the functional package manager.
* guix-package (guix)Invoking guix package.  Managing packages with Guix.
* guix-build (guix)Invoking guix build.      Building packages with Guix.
@end direntry

Ok, that's more than enough for one message ...

Best,
karl


\input texinfo
@c -*-texinfo-*-

@c %**start of header
@setfilename guix.info
@documentencoding UTF-8
@settitle GNU Guix Reference Manual
@c %**end of header

@include version.texi
@set YEARS 2012, 2013

@copying
This manual documents GNU Guix version @value{VERSION}.

Copyright @copyright{} @value{YEARS} Ludovic Courtès

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled ``GNU Free
Documentation License.''
@end copying

@dircategory Package management
@direntry
* guix: (guix).       Guix, the functional package manager.
* guix package: (guix)Invoking guix package
                      Managing packages with Guix.
* guix build: (guix)Invoking guix build
                      Building packages with Guix.
@end direntry

@titlepage
@title GNU Guix Reference Manual
@subtitle Using the GNU Guix Functional Package Manager
@author Ludovic Courtès
@author Andreas Enge
@author Nikita Karetnikov

@page
@vskip 0pt plus 1filll
Edition @value{EDITION} @*
@value{UPDATED} @*

@insertcopying
@end titlepage

@contents

@c *********************************************************************
@node Top
@top GNU Guix

This document describes GNU Guix version @value{VERSION}, a functional
package management tool written for the GNU system.

@insertcopying