Allow building manuals with Texinfo before v4.12.

Allow building manuals with Texinfo before v4.12.

 doc/lispref/intro.texi (Acknowledgements): Convert to ISO-8859-1 encoding.
 doc/lispref/makefile.w32-in (MAKEINFO_OPTS): Add --enable-encoding.
 doc/lispref/elisp.texi: Use ISO-8859-1 in @documentencoding.
 doc/emacs/ack.texi (Acknowledgments): Convert to ISO-8859-1 encoding.  Use
 Texinfo @-commands for non Latin-1 characters.
 doc/emacs/emacs.texi: Use ISO-8859-1 in @documentencoding.
 doc/emacs/makefile.w32-in (MAKEINFO_OPTS): Add --enable-encoding.