If we haven't transformed the program name, use the info files we've
already downloaded instead of letting the build regenerate them.

Index: Doc/Makefile.in
===================================================================
--- Doc/Makefile.in.orig
+++ Doc/Makefile.in
@@ -355,7 +355,11 @@ install.info: texi
 	if ( \
 	    sed '/^@setfilename/s|zsh|$(tzsh)|' \
 		< $(sdir)/zsh.texi > infodir/tzsh.texi && \
+	    if cmp -s $(sdir)/zsh.texi infodir/tzsh.texi; then \
+	        mv $(sdir)/$(tzsh).info* infodir; \
+	    else \
 	    (cd infodir && $(MAKEINFO) tzsh.texi) && \
+	    :; fi && \
 	    for file in infodir/$(tzsh).info*; do \
 		$(INSTALL_DATA) $$file $(DESTDIR)$(infodir) || exit 1; \
 	    done \