Prevent haddock docs being built when HADDOCK_DOCS=NO

rules/haddock.mk

@@ -21,7 +21,9 @@ else
 $$(error Already got a haddock file for $$($1_PACKAGE))
 endif
 
+ifeq "HADDOCK_DOCS" "YES"
 $(call all-target,$1_$2_haddock,$$($$($1_PACKAGE)_HADDOCK_FILE))
+endif
 
 $$($1_PACKAGE)_HADDOCK_DEPS = $$(foreach n,$$($1_$2_DEP_NAMES),$$($$n_HADDOCK_FILE))