--- a/config/makefiles/xpidl/Makefile.in
+++ b/config/makefiles/xpidl/Makefile.in
@@ -67,25 +67,33 @@ registered_xpt_files := @registered_xpt_
 xpt_files := $(registered_xpt_files) @xpt_files@
 
 @xpidl_rules@
 
 depends_files := $(foreach root,$(xpidl_modules),$(idl_deps_dir)/$(root).pp)
 
 GARBAGE += $(xpt_files) $(depends_files)
 
-xpidl:: $(xpt_files) $(chrome_manifests) $(interfaces_manifests)
+# We can't have the install manifest processing as an actual dependency of
+# the xpts because it is .PHONY and would cause the target to always be out
+# of date. So we have make reinvoke itself with another rule.
+.PHONY: xpidl
+xpidl::
+	$(call py_action,process_install_manifest,$(DIST)/idl $(DEPTH)/_build_manifests/install/dist_idl)
+	$(MAKE) xpidl-real
+
+.PHONY: xpidl-real
+xpidl-real:: $(xpt_files) $(chrome_manifests) $(interfaces_manifests)
 
 $(xpt_files): $(process_py) $(call mkdir_deps,$(idl_deps_dir) $(dist_include_dir))
 
 -include $(depends_files)
 
 define xpt_deps
 $(1): $(call mkdir_deps,$(dir $(1)))
 $(1): $(addsuffix .idl,$(addprefix $(dist_idl_dir)/,$($(basename $(notdir $(1)))_deps)))
 ifneq ($($(basename $(notdir $(1)))_deps),$($(basename $(notdir $(1)))_deps_built))
 $(1): FORCE
 endif
 endef
 
 $(foreach xpt,$(xpt_files),$(eval $(call xpt_deps,$(xpt))))
 
-.PHONY: xpidl

--- a/xpcom/xpidl/Makefile.in
+++ b/xpcom/xpidl/Makefile.in
@@ -1,10 +1,9 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 export::
-	$(call py_action,process_install_manifest,$(DIST)/idl $(DEPTH)/_build_manifests/install/dist_idl)
 	$(call SUBMAKE,xpidl,$(DEPTH)/config/makefiles/xpidl)
 
 clean clobber realclean clobber_all distclean::
 	$(call SUBMAKE,$@,$(DEPTH)/config/makefiles/xpidl)