Fix in tree build of protobuf
If we are building protobuf because of an incompatible out of tree protobuf installation, then make sure that we don't accidentally use that versions includes.
diff --git a/Makefile b/Makefile
index ae5d5d4..4efb955 100644
--- a/Makefile
+++ b/Makefile
@@ -315,8 +315,8 @@
ifeq ($(HAS_SYSTEM_PROTOBUF),false)
ifeq ($(HAS_EMBEDDED_PROTOBUF),true)
PROTOBUF_DEP = $(LIBDIR)/$(CONFIG)/protobuf/libprotobuf.a
-CPPFLAGS += -Ithird_party/protobuf/src
-LDFLAGS += -L$(LIBDIR)/$(CONFIG)/protobuf
+CPPFLAGS := -Ithird_party/protobuf/src $(CPPFLAGS)
+LDFLAGS := -L$(LIBDIR)/$(CONFIG)/protobuf $(LDFLAGS)
PROTOC = $(BINDIR)/$(CONFIG)/protobuf/protoc
else
NO_PROTOBUF = true