Check haddocking works when validating

validate

@@ -11,8 +11,9 @@ if [ -f mk/build.mk ]; then
 fi
 
 # The default is a "quick" build
-echo BuildFlavour=quick >mk/build.mk
-cat mk/build.mk.sample >>mk/build.mk
+echo BuildFlavour=quick >  mk/build.mk
+echo HADDOCK_DOCS=YES   >> mk/build.mk
+cat mk/build.mk.sample  >> mk/build.mk
 
 # You can override the default validate settings using mk/validate.mk
 # e.g. you could add GhcLibWays=p to test profiling.