Fixup for bug 1114669 on a CLOSED TREE. r=gps

xpt files don't have a dependency on backend files to avoid rebuilding all
of them when adding or removing new files. On incremental builds, some kind
of dependencies are required to ensure the xpt files are refreshed when
adding or removing new idls.

config/makefiles/xpidl/Makefile.in

@@ -56,7 +56,12 @@ xpidl:: $(xpt_files)
 
 $(xpt_files): $(process_py) $(call mkdir_deps,$(idl_deps_dir) $(dist_include_dir))
 
-$(foreach xpt,$(xpt_files),$(eval $(xpt): $(call mkdir_deps,$(dir $(xpt)))))
+define xpt_deps
+$(1): $(call mkdir_deps,$(dir $(1)))
+$(1): $(addsuffix .idl,$(addprefix $(dist_idl_dir)/,$($(basename $(notdir $(1)))_deps)))
+endef
+
+$(foreach xpt,$(xpt_files),$(eval $(call xpt_deps,$(xpt))))
 
 $(call include_deps,$(depends_files))