Makefile: Warn if policy.xml xmllint check does not run.

Signed-off-by: Chris PeBenito <pebenito@ieee.org>

.travis.yml

@@ -49,6 +49,7 @@ addons:
     - libbz2-dev
     - libpcre3-dev
     - swig
+    - libxml2-utils
 
 before_install:
   - lsb_release -a

Makefile

@@ -442,6 +442,8 @@ $(polxml): $(layerxml) $(tunxml) $(boolxml)
 	$(verbose) echo '</policy>' >> $@
 	$(verbose) if test -x $(XMLLINT) && test -f $(xmldtd); then \
 		$(XMLLINT) --noout --path $(dir $(xmldtd)) --dtdvalid $(xmldtd) $@ ;\
+		else \
+		echo "$@ XML validation not run. Please install the xmllint tool." ;\
 	fi
 
 xml: $(polxml)