Fix build with texinfo 6.7. Otherwise

    makeinfo -I topdocs --no-split --no-headers --output AUTHORS topdocs/AUTHORS.texi

fails with

    utf8 "\xC1" does not map to Unicode at /nix/store/...-texinfo-6.7/share/texinfo/Texinfo/ParserNonXS.pm line 1796, <FH> line 38.

--- a/topdocs/AUTHORS.texi
+++ b/topdocs/AUTHORS.texi
@@ -1,2 +1,3 @@
 \input texinfo
+@documentencoding ISO-8859-1
 @setfilename AUTHORS.info
--- a/topdocs/README.texi
+++ b/topdocs/README.texi
@@ -1,2 +1,3 @@
 \input texinfo
+@documentencoding ISO-8859-1
 @setfilename README.info