avoid hadrian/bindist/Makefile install_docs error when --docs=none

When docs are disabled the bindist does not have docs/ and hence docs-utils/ is not generated.
Here we just test that docs-utils exists before attempting to install prologue.txt and gen_contents_index
to avoid the error:

/usr/bin/install: cannot stat 'docs-utils/prologue.txt': No such file or directory
make: *** [Makefile:195: install_docs] Error 1

hadrian/bindist/Makefile

@@ -193,9 +193,10 @@ install_docs:
 	for i in $(DOCS); do \
 		cp -R $$i "$(docdir)/"; \
 	done
-
-	$(INSTALL_DATA) docs-utils/prologue.txt "$(docdir)/html/libraries/"
-	$(INSTALL_SCRIPT) docs-utils/gen_contents_index "$(docdir)/html/libraries/"
+	if [ -d docs-utils ]; then \
+		$(INSTALL_DATA) docs-utils/prologue.txt "$(docdir)/html/libraries/"; \
+		$(INSTALL_SCRIPT) docs-utils/gen_contents_index "$(docdir)/html/libraries/"; \
+	fi
 
 BINARY_NAMES=$(shell ls ./wrappers/)
 install_wrappers: install_bin_libdir