Bug 1239217 - Skip empty install manifests, apart a few exceptions. r=gps

Install manifests are not empty in normal conditions, apart a few
exceptions where they are only used for a "magic" `rm -rf`.

However, we're going to introduce changes that will empty some of
the install manifests and make their work happen from a different
backend, in which case we don't want them to correspond to a `rm -rf`.

Makefile.in

@@ -128,16 +128,13 @@ ifdef JS_STANDALONE
 NO_REMOVE=1
 endif
 
-.PHONY: $(addprefix install-,$(install_manifests))
-$(addprefix install-,$(filter dist/%,$(install_manifests))): install-dist/%: $(install_manifest_depends)
-	$(call py_action,process_install_manifest,$(if $(NO_REMOVE),--no-remove )$(DIST)/$* _build_manifests/install/dist_$*)
+.PHONY: $(addprefix install-,$(subst /,_,$(install_manifests)))
+$(addprefix install-,$(install_manifests)): install-%: $(install_manifest_depends)
+	$(addprefix $(call py_action,process_install_manifest,$(if $(NO_REMOVE),--no-remove )$*) ,$(wildcard _build_manifests/install/$(subst /,_,$*)))
 
 # Dummy wrapper rule to allow the faster backend to piggy back
 install-dist_%: install-dist/% ;
 
-install-_tests: $(install_manifest_depends)
-	$(call py_action,process_install_manifest,$(if $(NO_REMOVE),--no-remove )_tests _build_manifests/install/_tests)
-
 # For compatibility
 .PHONY: install-tests
 install-tests: install-_tests

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -391,19 +391,14 @@ class RecursiveMakeBackend(CommonBackend):
         self.backend_input_files.add(mozpath.join(self.environment.topobjdir,
             'config', 'autoconf.mk'))
 
-        self._install_manifests = {
-            k: InstallManifest() for k in [
-                'dist_bin',
-                'dist_branding',
-                'dist_idl',
-                'dist_include',
-                'dist_public',
-                'dist_private',
-                'dist_sdk',
-                'dist_xpi-stage',
-                '_tests',
-                'xpidl',
-            ]}
+        self._install_manifests = defaultdict(InstallManifest)
+        # The build system relies on some install manifests always existing
+        # even if they are empty, because the directories are still filled
+        # by the build system itself, and the install manifests are only
+        # used for a "magic" rm -rf.
+        self._install_manifests['dist_public']
+        self._install_manifests['dist_private']
+        self._install_manifests['dist_sdk']
 
         self._traversal = RecursiveMakeTraversal()
         self._compile_graph = defaultdict(set)