Splitting the info files doesn't bring much benefits these days.
This fixes also untracked generated info files from git ignore.

Let's use MAKEINFOFLAGS for common flags, --number-sections is already
the default anyway, so adding it doesn't change the info output.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170113144135.5150-18-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>