--- a/Makefile.in
+++ b/Makefile.in
@@ -91,47 +91,21 @@ config.status js/src/config.status:
 # The mach build driver will ensure the backend is up to date for partial tree
 # builds. This cleanly avoids most of the pain.
 
 ifndef TEST_MOZBUILD
 
 .PHONY: backend
 backend: $(BUILD_BACKEND_FILES)
 
-# A traditional rule would look like this:
-#    backend.%:
-#        @echo do stuff
-#
-# But with -j<n>, and multiple items in BUILD_BACKEND_FILES, the command would
-# run multiple times in parallel.
-#
-# "Fortunately", make has some weird semantics for pattern rules: if there are
-# multiple targets in a pattern rule and each of them is matched at most once,
-# the command will only run once. So:
-#     backend%RecursiveMakeBackend backend%FasterMakeBackend:
-#         @echo do stuff
-#     backend: backend.RecursiveMakeBackend backend.FasterMakeBackend
-# would only execute the command once.
-#
-# Credit where due: http://stackoverflow.com/questions/2973445/gnu-makefile-rule-generating-a-few-targets-from-a-single-source-file/3077254#3077254
-$(subst .,%,$(BUILD_BACKEND_FILES)):
-	@echo 'Build configuration changed. Regenerating backend.'
-	$(PYTHON) config.status
+include $(topsrcdir)/build/rebuild-backend.mk
 
 Makefile: $(BUILD_BACKEND_FILES)
 	@$(TOUCH) $@
 
-define build_backend_rule
-$(1)_files := $$(shell cat $(1).in)
-$(1): $$($(1)_files)
-$$($(1)_files):
-
-endef
-$(foreach file,$(BUILD_BACKEND_FILES),$(eval $(call build_backend_rule,$(file))))
-
 default:: $(BUILD_BACKEND_FILES)
 endif
 
 install_manifests := \
   $(addprefix dist/,branding idl include public private sdk xpi-stage) \
   _tests \
   $(NULL)
 # Skip the dist/bin install manifest when using the hybrid

new file mode 100644
--- /dev/null
+++ b/build/rebuild-backend.mk
@@ -0,0 +1,33 @@
+# 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/.
+
+BACKEND_GENERATION_SCRIPT ?= config.status
+
+# A traditional rule would look like this:
+#    backend.%:
+#        @echo do stuff
+#
+# But with -j<n>, and multiple items in BUILD_BACKEND_FILES, the command would
+# run multiple times in parallel.
+#
+# "Fortunately", make has some weird semantics for pattern rules: if there are
+# multiple targets in a pattern rule and each of them is matched at most once,
+# the command will only run once. So:
+#     backend%RecursiveMakeBackend backend%FasterMakeBackend:
+#         @echo do stuff
+#     backend: backend.RecursiveMakeBackend backend.FasterMakeBackend
+# would only execute the command once.
+#
+# Credit where due: http://stackoverflow.com/questions/2973445/gnu-makefile-rule-generating-a-few-targets-from-a-single-source-file/3077254#3077254
+$(subst .,%,$(BUILD_BACKEND_FILES)):
+	@echo 'Build configuration changed. Regenerating backend.'
+	$(PYTHON) $(BACKEND_GENERATION_SCRIPT)
+
+define build_backend_rule
+$(1)_files := $$(shell cat $(1).in)
+$(1): $$($(1)_files)
+$$($(1)_files):
+
+endef
+$(foreach file,$(BUILD_BACKEND_FILES),$(eval $(call build_backend_rule,$(file))))