Delay errors from missing 'xpath' executable

Currently running any make target without 'xpath' installed will print:
make: xpath: No such file or directory

This looks bad but isn't actually a problem unless you're trying to use
the zip-file target (or the lwg$(LISTREV).zip target it depends on).

Suppress the error for xpath not being found and only fail if the
LISTREV variable is empty when building the lwg$(LISTREV).zip target.

Author: jwakely

Makefile

@@ -65,11 +65,12 @@ history: bin/lists
 mailing:
 	mkdir $@
 
-LISTREV := $(shell xpath -q -e 'substring(issueslist/@revision,2)' xml/lwg-issues.xml)
+LISTREV := $(shell type xpath 2>/dev/null && xpath -q -e 'substring(issueslist/@revision,2)' xml/lwg-issues.xml)
 
 zip-file: lwg$(LISTREV).zip
 
 lwg$(LISTREV).zip: lists
+	@test -z "$(LISTREV)" && echo "Install 'xpath' to create zip file" >&2 && false
 	@cd mailing && zip ../$@ \
 		lwg-active.html lwg-closed.html lwg-defects.html \
 		lwg-index.html lwg-index-open.html lwg-status.html lwg-toc.html