Extra protobuf-related support

Change-Id: I375d32f0bcb695ccb0603743133034941d584163
diff --git a/core/definitions.mk b/core/definitions.mk
index ac431df..36285b8 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -770,6 +770,7 @@
 $(hide) $(PROTOC) \
 	$(addprefix --proto_path=, $(PRIVATE_PROTO_INCLUDES)) \
 	$(PRIVATE_PROTO_JAVA_OUTPUT_OPTION)=$(PRIVATE_PROTO_JAVA_OUTPUT_DIR) \
+	$(PRIVATE_PROTOC_FLAGS) \
 	$(PRIVATE_PROTO_SRC_FILES)
 $(hide) touch $@
 endef
@@ -782,6 +783,7 @@
 @echo "Protoc: $@ <= $<"
 $(hide) $(PROTOC) \
 	$(addprefix --proto_path=, $(PRIVATE_PROTO_INCLUDES)) \
+	$(PRIVATE_PROTOC_FLAGS) \
 	--cpp_out=$(PRIVATE_PROTO_CC_OUTPUT_DIR) $<
 endef