Fix doc build inconsistency

Remove the doc targets completely not partially when disabled.
Otherwise even though we are able to build the targets but we will see
some strange errors.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2470

ghc.mk

@@ -712,6 +712,7 @@ BUILD_DIRS := $(filter-out utils/haddock,$(BUILD_DIRS))
 BUILD_DIRS := $(filter-out utils/haddock/doc,$(BUILD_DIRS))
 endif
 ifeq "$(BUILD_SPHINX_HTML) $(BUILD_SPHINX_PDF)" "NO NO"
+BUILD_DIRS := $(filter-out docs/users_guide,$(BUILD_DIRS))
 # Don't to build this little utility if we're not building the User's Guide.
 BUILD_DIRS := $(filter-out utils/mkUserGuidePart,$(BUILD_DIRS))
 endif